Thanks for your reply.
o.k. so PM cannot use another table, but pm1_users...
What about renamimg pm1_users to a different name and creating a custom view like:
CREATE VIEW pm1_users AS SELECT
IDUser AS UserID,
NameUser AS UserLastName,
EMailUser AS UserEmail
After some testing I got this running by creating a view, similar to the one above. The custom view seems to need all the fields from the original pm1_users table. So, adding these to my users table with "NULL" as default value, worked. Powermessenger simply expects the fields to be there. Those column names from my users table, that were different to those from pm1_users must be selected like: "IDUser AS UserID" to correspond the correct names.
It might need some further testing, because, for example, the unsubscribe links from tokens would not work. I guess the easiest way would be to add a custom unsubscribe page with DataBridge...
But for my purpose, to integrate PM into an existing site with another "users" table, this works perfect.