Contact the cable company to see what they offer for condo/apartment complexes that require a "lobby watch" channel. There should be some sort of route.

Some issues to consider:

1) Digital modulation - even if you can find a random QAM modulator, the cable company's cable boxes will NOT tune it. QAM-enabled televisions would, though.

2) Lack of free channels - most digital cable systems use every single available channel, which means you need a device to clear off a channel before you could inject anything back in. Doing that will of course remove some channels, either a single analog or multiple digital.

3) If you go the analog route - do the client's digital cable boxes even tune in analog channels? I know where I am there are like 60 analog channels in the system, but the cable company's cable boxes only tune in digital versions of them. So replacing analog channel 5 has no impact on what shows up as digital channel 5, and there's absolutely no way to see the analog version with the box.

I have all the official equipment necessary to do this in my house for digital cable - it involves a device to strip off digital channel 59 (which is occupied with some foreign digital channels), an analog modulator to create a new analog 59, and a combiner to merge it all together. On the digital cable boxes this shows up magically as channel 999 "Lobby Watch", however it ONLY works if the box supports analog tuning.