A line-level signal is around 1V and a microphone signal is around 10-100mV (depending on the loudness and how close the line level is to the home audio or pro audio standards, etc.). Headphone-level is also close to line-level, so you can connect a headphone output to a line-input. (But, not the other way around... The voltage is OK, but the headphone impedance is too low for a line-output.) So, you can use a voltage divider (2 resistors) to knock-down the signal to somwhere between 1/10th and 1/100th of it's original voltage.
You can experiment, or use a pot to make it adjustable.
But, that shield looks expensive! I'd be cautious about hacking it up!
Also, since it looks like you'll be transmitting mono you'll need to mix the left & right channels together. Do NOT hard-wire the channels togther. You should NEVER connect two outpus together. (It's usually OK to connect two inputs together.) Put a resistor (1K - 10K) in series with each output. (Those resistors can be part of your voltage divider.)
And it's not a big deal in this case, but in-general, you'd like to avoid reducing line-level to mic-level whenever possible. All amplifiers add noise so there is added noise when you re-amplify the signal.