I want my portal to use our existing customer database.  i.e. when a new user registers on the portal I want a record to be inserted into our Customer table which is in the same SQL Server database as the DotNetNuke tables.
I am thinking of doing this with a Trigger on the Users table.  i.e. when a new record is added to Users, the trigger will fire to create a new record on the Customer table. 
1/ Is this recommended or is there a better way?
2/ I have an issue. On the Customer table there is a column called 'StoreNo' with a unique constraint on it.  I have added a similar column to the Profile in DotNetNuke so the data can be captured but I need a suitable error to be shown in the portal if a user tries to enter a duplicate StoreNo.  I can stop the User record being created in this case by doing a rollback in the trigger but don't know how to make DotNetNuke show the error.  Is this possible?

Thank you - do you know how I display an error back in the portal if the data entered by the trigger fails validation on the Customers table?