mix 'n match
My buddy had to deal with his cable system, which was pretty much full up.
He wanted to put three satellite receivers on-line for the house, and share them with the cable channels.
Radio Shack was discontinuing some stereo digital modulators at the time.
I think they were priced around $20 or 25, and tuned VHF, UHF, and Cable frequencies.
They had an LED 7-segment display, and up/down push buttons to select a channel.
(I've seen similar units at Sadoun, or search eBay/internet)
Forget the details, but I think we put the modulators on 81, 83, & 85.
We ran the three modulator outputs -and- the cable signal, backwards through a 4:1 TV splitter.
Luckily, the amplitudes were close, and it worked pretty well.
In testing, we found that the cable had noise in the range of our modulator channels, and caused some interference.
We got some sort of low-pass filter that cut off above channel 80, and pre-filtered the cable signal.
That cleaned up the interference and the combined signal got distributed throughout the house.
Later, when converter boxes became available, one of the satellite channels gave way to local channel 18.2, the all-Japanese channel!
(it wasn't on the cable, and it wasn't on the satellite)