Freezing user sessions and switching to another account.

Hello everyone,

that's my first post on the forum. Could anybody help my solve this problems?

1. I need receive password from user and concatenate with constant string and then change password in system that user could log in this account only through my transaction.

2. Then, if necessary user calls my transaction, types his part of password, and he's switched to that set up account, during he uses this special account his previous sessions are froze.

Thanks a lot in advance.