In English, there are only a given set of phrases that mean "in bocca al lupo." A few of them are:
Lots of luck
Break a leg
There are ways to manipulate these to make puns or variations, but clearly "in bocca allo zombie" is a variation of the very unique phrase "in bocca al lupo." If the person still means "good luck," then you must either use one of the set phrases in English, or you can come up with your own variation on one of them...or scour the internet to find variations.
Hmm... well, it's difficult, because the closest thing we have to 'in bocca al lupo' is 'break a leg', or just 'good luck'. Because of its origin, you can only really use zombie with it still making sense in Italian. It depends on the context how important the 'zombie' part is, but if it is meant to be as silly as it sounds, then I guess you could get away with something like "break a zombie leg"! Not that it would make much sense...
Knock 'em dead
May the force be with you
God speed (Pretty fancy and epic)
Here's another option that plays off "break a leg" that might capture the essence of the original:
Break a femur!
The femur is the thigh bone and is generally only referred to as a "femur" when people talk about it being broken. It carries with it a gruesome connotation, so it's a slightly different twist on a strange old way of saying "good luck."