Could you use a DAC to scale and modify an input signal?
For many D/As, yes that works.  Sometimes devices were designed deliberately for this type of operation and called Multiplying DACs.

Check the bandwidth and delay from reference to output carefully.  These might not be what you expect from looking at the number to analog value delay spec.

Although you say "modify" in your question, the only thing such a DAC can do with your signal is attenuate it, with some unavoidable delay and low pass filtering.