I'm not 100% sure I understand your set up, but you can always set up the Slingbox as a "Pass through" device... in other words, you can stream from just about any video source, and pass that video source through the Slingbox along the signal path.
This article is a little older but outlines the premise of pass-through audio/video: Slingbox.com - How audio and video passthrough works on your Slingbox
I have just purchased my first slingbox. My cable tv runs through a DVD player into the TV. The tv is wall mounted . Can I install the sling box in between the Cable box and the dvd player? Also, the DVD player and TV have fewer inputs than the slinbox has outputs (these are an older TV and DVD player). How can I deal with this issue?