RC controller stick input bypass

Hey everyone!
I need some input on how to make this idea I have happen with an Arduino. I have a couple Pro minis lying around.
What I would like to do is automate certain stick movements on an RC transmitter so that the person operating it doesn't have to hold a certain position long, while keeping the placement accurate. I have taken it apart and the stick in the "untouched" state has about 1.60V, moving it to one side causes the voltage to go down to max 1.35V, or right to max 1.85V (iirc).
What I'm usure about is how to bypass this, but without disabling the physical sticks.
So for example there would be a small LCD and buttons wired to the Arduino. Using the buttons, I would set speed forward of 4 (ie. on a scale of 1-10). This would translate to setting 1.7V on the certain controller pin. I would then press a "Go" button and the human stick movement would be emulated until a "Stop" button is pressed. BUT in case I wanted to physically speed it up with the stick, it would do so, above 1.7 to 1.85 logically.
From previous projects, I am guessing I would be using analogWrite with the 0.50V (1.35-1.85) converted to lets say 100bit resolution, where 50 would be zero movement and every + or - 1bit would mean 50 increments of movement in either direction (much better than a human hand can hold steady). Imagine an RC car that can keep going around in a steady "perfect" circle at a steady speed. Not accounting for physical characteristics like wheel camber etc., not yet anyways.
The gadget would be mounted to the controller, not into the car as I have seen in some videos. This way I would use the current Tx and no changes would be made in the toy.

In short, what I'm asking for is just a nudge regarding the lowering and increase of the voltages using the Arduino, without corrupting the voltages from the sticks. I think I have enough IDE experience to put this together after that.

Edit: Is should add that I don't want the sticks to be physically moved by the arduino via servos for example. When the gadget is operating, emulating a movement, nothing is happening with the sticks, they're in their default center position.

A 74HC4053 will let you switch three channels, individually, from manual (stick) to Arduino control. So with one digital pin on the Arduino you can switch between the signal from the stick or the signal that your Arduino is creating. A 74HC157 will let you switch four channels, all at the same time, between manual and Arduino control.

The difficulty here is smoothing out the analogWritten PWM (square wave) signal from the Arduino into something smooth enough that your receiver will accept it. I'm going to defer to others on the forum for suggestions on how to do that.

I don't think simple switching between an Arduino and the joystick potentiometer will allow the sort of behaviour you describe (if I understand correctly). You have described a situation in which the Arduino holds a minimum setting of (say) 20% which you can override with the 'stick but only above the minimum. If you switch back to the 'stick it will go back to its normal 0-100% behaviour.

My feeling is that you need to feed the stick output into the Arduino and let the Arduino produce a suitable output that the transmitter thinks is coming from the stick. You could have the Arduino control a digital potentiometer in response to a combination of the stick input and the current preset position.

Another option might be for the Arduino to intercept and modify the pulses produced by the transmitter but I have no idea how or whether you could access that data within the transmitter circuit.

...R

How about using a micro servo or similar to adjust the trim? Usually this works by physically moving the body of the potentiometer. You may need to hack the mechanism to achieve a sufficient range of motion. Alternatively if you only need a few discrete movements you could use a micro relay to connect a resister in series with the pot to bias the pot output. I don't know how much current the pots take, but if it's of the order of 20mA or so you could even use I/O pins directly to add pull-up/pull-down resistance to the signal pin.

PeterH:
I don't know how much current the pots take, but if it's of the order of 20mA or so you could even use I/O pins directly to add pull-up/pull-down resistance to the signal pin.

I like this idea. I would basically be lowering/increasing the voltage however needed, between the pot and the transmitter, correct?
In this case, if I lowered voltage by .25V (= "emulated" stick 50% left from center) and then physically went al the way left with the stick, would the 50% to 100% not being Arduino emulated still work physically? Its hard to explain.
I'll try to find out how much current is going through the pots, but my guess is somehwere around 20mA. I have 4 1.5V AA 2700mah batteries in the Tx and they last ages. Most current is probably doing to the transmitter.

I'm trying to stay away from microservos to keep it as simple with low number of added parts.

Or possibly I could use something like this Digital Potentiometer - 10K - COM-10613 - SparkFun Electronics
to change the voltage digitally.

Anyone have any thoughts on the digital pot?
What I would like to do is digitally change the stick's voltage range, ie. 1.35V and 1.85V by just tapping into that one signal lead before the transmitter.