Ooer, seems odd as an RC mod - most toy car enthusiasts struggle soldering wires to motors, let alone decyphering a PCB to find the correct place to tap in a conversion board! Bit of a "get-out" really, cos that kind of mod has always been about if you have the skill or enthusiasm to do it (eg 402 Tx/Rx boards for telemetry & older car alarms). Think I'll stick with my (Simpler) method for now then.
There are 2x reasons I mod trannies - I don't get on with pistol types, which seem to be cheaper, more available, & in some cases, the only type you can get with that particular RTR (eg Radline). Other reason for modding is to add a channel or 2 to 27am stuff, cos that's what most of my gear is, & although you can easily lay your hands on cheap chines 4ch 27am RXs, there's a distinct lack of 4ch 27am TXs. Do the mod right & your new 4ch will still work properly with 2ch RXs too, so it also cuts down on the number of TXs you have kicking about.
Yes, normal scenario is to rip the board out of a pistol TX, then set about shoehorning it into a stick case, more often than not a cheapy chinese one, but it depends what cheap job-lots of busted trannies are on Ebay. Don't touch those with missing battery covers or aerials, as these are non-starters with the cost to repair to use. Then, it's just a case of matching the restive values of the gimble-pots & transferring their wires over. Sometimes you have to remote-mount things like the crystal socket & servo reversing switches - remove them from the PCB, mount them somewhere convenient & wire them back to the PCB from whence they came, pin-for-pin. I haven't had any issues remote-mounting crystal sockets, but it's wise with sensitive stuff like that to keep the wires short, away from large currents or strong signals & use grounded shielded cable if poss. As I said, the only things you need to be careful about are the pot values have to be the same as the originals, & the resistive values at full stick movements need to be the same too. Last one of these mods I did was a 2.4GHz Hobby King (HK2?) pistol PCB into some undefined chines "toy" 2ch heli stick case. Aerial lead had to be removed & extended, which was a bit of a faff due to the surface-mount components & multi-layer minature board, trim/dial/LED panel was remote mounted as a lump on the front of the tranny, er, power switch was removed & wired to the existing one, pots had to be replaced to get the right overall & travel values - steering on the HK was full-scale of the pot, whereas sticks only move it through 60deg, so a bit of messing about with separate resistors there to get the overall resistance right AND the correct resistive "sweep" in the 60deg stick movement.
Adding channels - only possible/easy with a particular type of transmitter, or rather, those that use a chip rather than individual components to do the work. MSL9362 is the chip number, it's designed for operating 4ch, but is often found in 2ch TXs with the extra 2x channels blanked, all you have to do is add a couple of components & modify the PCB tracks a bit (The unused channel inputs are usually either hooked to ground or the +ve supply rail, so you have to isolate them to be able to use them). Off the top of my head, I know Traxxas TQ3 has this chip, & uses ch1,2 & 4, 3 is grounded & can be isolated & used with a pot as a proportional channel, or a switch & used as digital. Ch4 is wired digital (For forward/reverse gearchange on the TMaxx, or 1st/2nd gearchange on EMaxx) but can also be wired to a pot instead for proportional. Another tranny with this chip is Acoms Techniplus Alpha, although I can't confirm this as I've not got around to ripping mine to bits yet! Apparently the Lithuanians use/convert them for use with 3ch/4ch gliders
