Actually, you CAN. As long as you have NR2003 selected in the WinMip2 menu, you just have to go to extras>NR2003 car file editor. It will bring up the file selection screen and you can load in any carfile you would like. You can then import any flat onto the car directly outside of the game. I have noticed a bug, however, that prevents one from importing a pit crew to the carfile, so you would have to go into NR2003 and import them with that method. In between the flat previews, you can change the bit rate as well as if the file is compressed or not* as well as giving the option for whether you want to import a tga file or a bmp file.
In the lower right half of the editor there is a text box with information pertaining to the car file you have loaded. From there you can change the usual things such as driver name, their car number, etc. However, you can also see and change stuff you normally wouldn't see in NR2003 like what "Class" of car the file is (1=Cup, so I would assume 2 is GNS, 3 is CTS, and 4 is PTA) as well as the model of car (0-3, 0=Chevy, 1=Dodge, 2=Ford, 3=Pontiac). There's also additional info such as a driver's career stats or where their hometown is, though any changes you make to those are useless as NR2003 ignores those. You can also theoretically change the driver ratings in the same area as well, but I haven't played around with the AI parameter stuff as NRatings exists so there's really no need to.
*ALWAYS COMPRESS THE CARFILES (and import through WInMip2 as much as possible)! NR2003's internal Import/Export system bloats the size of the files modified within.