Foster arrives at the club from Exeter Chiefs, while winger McCrea has agreed a one-and-a-half-year deal from Connacht.
"Mark Foster has won this league before with Exeter and has a great deal of experience in the Premiership and the top end of the Championship which will be invaluable at this stage of our campaign," Harvey told the club's official website.
"Mark McCrea is at an earlier stage in his career, and he has the opportunity to get regular game-time over the next 14 months and become a key part of our backline.
"Both these signings are quality players who I expect to make a significant impact in Jersey colours."
Both players could make their debut against London Scottish at the weekend.