if you already have the member created that you want as an admin, just run the stored procedure
dbo.aspnet_UsersInRoles_AddUsersToRoles.
DECLARE @return_value int
EXEC @return_value = [dbo].[aspnet_UsersInRoles_AddUsersToRoles]
@ApplicationName = N'YOURAPPLICATIONNAME',
@UserNames = N'USERNAME',
@RoleNames = N'ROLENAME',
@CurrentTimeUtc = N'CURRENTDATE'
SELECT 'Return Value' = @return_value
hope that helps