Porting is not difficult but you may want new sprites depending on how far you want to push your quality. This also assumes you have a sprites.png available.
RR has 2 new sets of sprites that are needed, those being lookback and death sprites. You need to create these in order to port the character over to Ring Racers.
Porting is as easy as to just grab Ring Racers under kartkrew.org that comes bundled with a kartmaker folder. If you have used it before, simply copy paste the sprites meant for their state into the template like before. For the new slots, either draw them yourself or just copy paste further if you don't really care. Then create the wad. If you have not used kartmaker before, are enough guides out there explaining how to use it.
Ring Racer kart sprites are roughly 10% bigger than before, so if you don't intend on redrawing and just use the old kart from SRB2K you may want to go into SLADE after creating the WAD, and define HiResScale = 1.1 into the S_SKIN text file. Otherwise, they may appear too small compared to the rest of the racers.