Updating SQL Server 2008 R2 using slipstreamed source
I don't know of a way to do this. When you build the slipstreamed install, it is no longer an SP1 installer, it simply updates the files that the initial setup program uses during installation. It does not build an RTM installer and a separate SP1 installer that are somehow daisy-chained.

So I think the answer is: you will have to download SP1 again, and run it separately from any media where you have created a slipstreamed install.