I think you're right.

First of all, some slingboxes have built-in tuners -- they can take an antenna feed. More likely, however, you were getting your video from a video out on the box. Same answer applies -- I doubt whether any random box will work.

For those devices that can take an antenna in feed, you can get splitters that will take the single feed and give you multiple feeds. Note, however, that you will lose signal strength by doing so.

If you're talking about devices that don't have an antenna input and require line-level video input, you're out of luck. Maybe you can find one of those replacement DTV tuners that tunes both NTSC and ATSC and has a video output.