You're using a lot of words like "inject" and "ramp" which don't make a lot of sense, so let's try to be clear: the map function takes several arguments, does some math, and returns a value.
Second, you're saying you want to make a button output a voltage, which doesn't make sense either -- buttons are inputs.
So what I think you mean:
You want to make a project that reads a button and outputs a voltage if the button is pressed (if it's not pressed, output 0 volts). The voltage depends on how long the button has been pressed; increasing as it has been held longer.
If that's so, you'll want to make it so that when the button was not pressed last time but was pressed this time, it resets a clock (or sets a variable to now). Then you want to map the elapsed time (now - the stored time) into a voltage that you feed to analogWrite.
now can be figured out using the millis() command. You can use a variable declared as static to keep a variable (such as the time when the button was first pressed) between iterations of a loop. The rest I think you know how to do.