Yes, theoretically that's it. However I'm not convinced that it's practical to create an add-on for SDRs that's not designed to be used as a transmitter. Usually these are single (or 2) chip solutions. I don't expect that you can reuse many things from the circuit. Probably you'll be developing a complete transmitter.
HackRF can transmit btw. I'm pretty sure that some high-end SDRs can transmit too, but I haven't used any of them.