Digital cable TV converter?

Let's clear up the terminology, because your box doesn't work that way. The ATSC converter box, at least the ones that were subsidized by the government have nothing to do with anything cable related, ie: They do not detect nor convert any QAM (cable) carriers at all. The converter ONLY deals with Off-Air ATSC carriers, and this was stipulated by the government since they paid for them. Your ATSC converter may pass analog cable signals, but it doesn't convert digital QAM (cable digital) carriers, scrambled or not. If your Radio Shack box operates any differently from this, you bought the deluxe version, because this is not how the run of the mill boxes operate, nor what the government spec'ed when they funded the program.

There are plenty of hybrid cable systems out there that contain analog and 'clear QAM' , and private systems built like this as well, I've built 4 of them. There's nothing to prevent a CG to add Clear QAM to an existing analog system.