Is one allowed to hire idolaters, if it is known that part of their salary is donated to idolatrous activities?
Hiring an idolater to do work for you is the same as purchasing something from an idolatrous merchant. The Sources are full of examples of the latter, which is obviously allowed. So the former is allowed also.
(To be sure, there are restrictions on trade with idolaters around the time of their festivals. They are described in tractate Avodah Zarah (Ch. 1), but they are insignificant in the big picture of trade with idolaters.)