Careers are a bit more than a single skill ore even a set of skills: they are a bunch of advancements (rank in skills, the WFRP equivalent of talents and so on) and some special equipment.
All those things are tied to a story.
The beset thing to do, in my opinion, in order to mimic WFRP, is this:
1. make 'No Profession' the only profession available for PCs.
2. introduce training packages from RMFRP.
3. spell can only be acquired from specific training packages.
So each PCs move from one training package to the next as in WFRP.
It could be done but it is too much work: I prefer the traditional professions.