Adding List property to ASP.net MembershipUser object

I have created an ASP.net MVC4 application with Internet application template, and want to add List<T> property to MembershipUser object provided by ASP.net builtin Authetication.
  This List property will be a list of StockPermission object:
public class StockPermission {     private Stock stock;     private byte afterSaveAction;     private bool allowEditingStockDocs;    private bool allowChangingUser; }
Therefore my MembershipUser will contain StockPermission objects which let MembershipUser to perform defined action with those Stocks in the List  
That means I want to create a custom MembershipUser which will contain property like this: List<StockPermission> StockPermissions

  Is there a way I can realize it? < ...