Is there a way to ask migrated users to complete their profile? I hope to migrate users in the database (using Drupal 7). 

How will I handle the fact that they have no password, and will need to create one before accessing their profile? 

Password is currently on the Join Form and these migrated users shouldn't have to go through the Join form.

Also - what do you suggest for mass email?

Thanks!
You can take help of Mass Password Reset module.

I had same kind of situation when I made a existing website of D6 into D7 &amp; migrated users from there.