Quoting The "p" word.:" If you were a nurse at an abortion clinic, what were some of your roles during an abortion? Did you have ... [snip!] ... them do it, but if it weren't for one nurse I probably wouldn't have agreed to do the abortion. I applied her for her service.."
A nurse technically can't be fired for not wanting to assist abortions, but I can't see a nurse applying to work at an abortion clinic.
But yeah, you would have to sign something in the contract stating that you are against helping out with abortions.