It's not clear exactly what you're trying to do, but I can't see any reason why switching to md5 would help you.
md5 is much less secure than sha1, and as pupi said we're switching to bcrypt in Q2A 1.8 because sha1 is also quite insecure nowadays.
If you need to use Q2A logins on custom pages outside of Q2A you can use Q2A functions to do so. See here for details and examples: http://docs.question2answer.org/code/external/
If you need to use some login code from an external app or other system, check out the Single Sign On feature: http://docs.question2answer.org/install/single-sign-on/