Master/Details

Is it possible to get a Master/Details page using the ASPNETUsers table as the Master?
I was trying to make a DataGrid of the Users (with a few columns) and the DataList to display all records from the related table Opportunities. Trying to do this in the CRM Demo by making a new page and selecting Master/Details. I'm not positive but I think a one to Many is not defined in the app but there is a one to many relationship showing in the database. If I'm correct how and where would I create the one to many so it would get over written?

Thanks