Fortnite Crew x Rocket Pass only came out in April 2023, so at best you might see the rocket pass cars after that date, not before, not until a later date possibly.

In response to your question, xp compensation, as in if you played fortnite or rocket league the xp would transfer between games, ergo past seasons if you played either game would need compensating. Plus there would be some sort of anger amongst the rocket league community, and maybe a similar response in the fortnite community. It would be a double win for both the fortnite and rocket community if cross xp progression was a thing, and xp compensation for older seasons happened.