Keep in mind a few things.
A) WFRP is an Empire-centric (i.e. human) game. 3rd Edition more-so than the previous ones, but even the first two editions were the same same when they came out. As such, careers for non-humans are relatively restricted.
B) Elves worship different gods than the human do. They have different blessings, etc. The core set, being Empire-centric, has no room nor place to delve into the Elven Pantheon, nor to provide a deck of blessings for elvish gods/goddesses.
So, while I don't believe that an Elven priest career is impossible, it should not be expected to be handled in the current state of the game. When there is an Elven expansion, I expect there will be at least some form of elven priests. Until that time, you'll need to make a homebrew version, including a deck of blessings, or else disallow elves from becoming priests.