Use password field on inactive account for identity confirmation

Hello,
I have a strange use case that many may think sounds silly, but the idea was put out to me by the client. Before I say no to them, I wanted to at least verify whether its absolutely not possible.

I'm doing a big community site which contains many users, some active, some inactive, who are imported from vBulletin – user accounts, passwords and all. They want old inactive members who are coming back after their account is inactive to be able to re-enroll using their old account (to prevent having account doubles). However, they insist on there being a security question to prevent a malicious person (like a disgruntled ex) from hijacking the account. Since there's no sure fire security question data available from any data migrated over (like What was your favorite pet?), they suggested I use the old password imported from vBulletin.

So the use case is that on the sign up page, there's an inactive account search. Once you pull up the old account, you have the option of using it for your user account if you answer the security question. This is where I'd want to be able to place a password field input where they'd enter their old password and have it verified, without actually logging into the site.

I don't know if I can even migrate the vBulletin password into a different field other than 'pass' in the 'user' table, for instance using the Password Field module, or if it's worth the pain. I figured I just need to see if its even possible to have someone enter a password into a field and, maybe with hook_form_alter intercept the password to determine something as simple as showing or hiding something in the display.

Any help on this would be appreciated.

Thanks