To add more gears to a car, you check to see if there is a gearbox with more speeds, or leave it up to the mods. There are many, many mods that will allow for more transmission options.
if you do want to mod one though, find the .jbeam file named "..._gearbox" or "..._transaxle", make a copy of it in mods/unpacked and copy the file path. Then, open your copy of the file, and scroll down to the "gearbox" section, then just add your desired ratios to the "gearRatios" line. the section above that labelled "powertrain" determines what type of gearbox it is (manual, auto, DCT, sequential, etc.), if you want to change that, just look at what other cars gearboxes use and copy accordingly, (bearing in mind you'll need a clutch for a manual, a torqeConverter for an auto, etc.)