I don't unsterstand very good your problem but I can explain how I proceed for my Porsche 997:
1º I paint the rims and save it in DDS DXT1.
2º You need to change the 3th Genstring number (rims number). In the Porsche, if you need BBS Rims the number are 1 (Silver) or 3 (Black). I put 3.
3º Later, on the VEH file too, you can see anything like this:
Now you know the 3th number on genstring are rims. If you put 3 (for example) your rims are black. For modify this to personal rims, you need to change the "Extra8="BBSBLACK"" line to "Extra8="BBSPERSORIMS"" or another personal name.
4º Finaly, you need to rename your DDS texture and put it in your team folder. Rename that to CARNAME_NUMBERANDRIMSNAME.dds.
Example: My car is the Porsche 997. I' paint my rims and save it in DDS (DXT1). On genstring I put BBS black rims (number 3). On the "Extra8" line (this line define dds texture name) I wrote "BBSAKRACING". My car number are 34 and year 2009. The final DDS name are: Porsche997_091034BBSAKRACING.dds.
I hope this help you and sorry for my english (I'm trying to speak whithout google trad ^^).