question about ASPNETDB stored proc

I am using the membership profice service which created ASPNETDB.MDF.  I am going to add a field to the Users Table and add data to the field upon Create User Wizard.  My question is how is the best way to add the data keeping in mind that the Create User uses a stored proc.?  Should I alter the stored proc?  should I just use code behind?  can I use SQLDataSource with that one field?  Is the stored proc fired first then a SQLDataSource (markup) is fired?  Is it vice versa? 
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