Migrate users & preserve passwords

I am a new user of ARRA User Migrate. I found this application really easy to use. It appears to work exactly as described.

I am trying to migrate users from a Joomla 3 site that developed a bug. The User Manager no longer displays the option to Assign Users to Groups, for example on the New User page.The only way I have found to solve this problem (so far) is to rebuild the site.

I can migrate users from the damaged site successfully using ARRA User Migrate; however, I do not know how to preserve passwords that accessed the old site. I want very much to avoid requiring all users to reset their passwords on first access to the new (i.e., rebuilt) site.

Any guidance about how to preserve passwords will be very much appreciated.

Again, thanks for a great application.