In addidtion to Jason's post, I've found a very helpful tutorial for this issue:
What I personally do to check if the import will perform correctly, is to make a copy of the "pm1_users" table in PHPMyAdmin first, to let's say "pm1_users_test".
Then perform the import into this table and see, if PHPMyAdmin throws any error messages.
If everything is o.k. PHPMyadmin will show you a message, of how many entries got inserted into your test table.
Then repeat the import procedure with the "real" pm1_users table...
You wouldn't even need all table columns. In my case, I get the addresses as xls-file from my customer, only containing UserEmail, UserFirstName and UserLastName.
So I only import these three columns into my pm1_users table, however adding a value to the password column on insert.