Where to store cross-reference data

I am wondering how/where people carry cross-reference data when using the ASP.Net security features.  We need to associate a CustomerNumber with each user and use the CustomerNumber to access the appropriate data for that use in another database.  Hence, we need a cross-reference between the user (aspnet_Users.UserId) and the CustomerNumber associated with them.

Adding a column to the aspnet_Users table doesn't strike me as a smart idea.  Is this normally handled by adding a new table to the database that contains the UserId and CustomerNumber?

Thank you.
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