There are profile preferences copied from Site Settings.
You can change them for the imported users as seen on the screenshot below.
One additional preference setting is Short password suffix.
Older versions of DotNetNuke used to have default setting of 4 characters long passwords. The required password length in current versions is 7 characters.
There are two workarounds for this problem. You can change (lower) the minimum required password length by changing web.config entry or you can have the module add a string to every password that's too short.
