hi,
Since the standard remotes are usually 315 mhz, may be kinda hard to make another one. Instead you can use a relay board and connect its switches in parallel with the buttons on the remote. I have done this before. solder the wiring inside the remote, make a little hole on the side of the plastic, glue a tiny female connector. then you do the rest outside, and connetc to it.
Let me know if i wasnt very clear