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Yes the majority of cable STBs do have an RF in. They certainly have an RF out that would transmit the cable output as well as via SCART out sockets.
Even if there is no RF in, you could just get a "Y" splitter, put that on your connection into the VCR so one in on the Y comes from the STB, the other in comes from the terrestrial aerial and the out from the Y goes into the VCR RF in.