It would be _possible_ to use the existing database for the PowerStore 3 registration system, but it will not be _simple_
the key to making it work is in the names of the columns.
look at your siting user table, and the power store user table, pay attention to the column names.
all of the power store code is going to be looking for those column names.
if you table does not use the same column names, the power store code will need to be modified to use the column names from your table. you really wont be able to do a simple find and replace, it will be more in depth than that.