run RSRBR in offline/single player mode.
select the car pack you want to use.
run RSRBR SSE.
please note you will be restricted a single car from each pack, and i believe some packs may not work due to where the cars are placed in the list or something.
outside of that you could make a second install and copy over the WRX with the car you want to drive, not sure how you would go about it tho.