pro-rated

[mato]

'a pro-rated holiday entitlement of...'

obviously pro rata is an adjective or an adverb, but can one pro rate something?

Don't think so..

"Holiday entitlement is pro rata"

Pro rata is Latin so you can't just add "d" on the end.

Strictly, no ... but for me this falls into life being too short to stuff a pedantic mushroom territory. It works, it's useful, everyone knows what you mean (and that you are stretching the grammar) - it's not so much an error as a liberty.