You can sign into the iPad with any AppleID.
AppleID's cannot be merged or transferred, at least according to their info on the AppleID website.
You can try to recover the other ID from here: https://appleid.apple.com/cgi-bin/WebObjects/MyAppleId.woa/
Click the Reset Password link under Manage your AppleID
Which AppleID did you use to sign into iCloud on the iMac? And how, if you don't know the password?