Can I add an SMA tap to BNC HF transceiver?
For speeds under 50MHz transmission line effects are usually not a problem for the short distances on the PCB above. The SMA connector would add a small capacitive load (probably in the pF's range, if your fine with that then I'd solder it on. I would not plug in two things at once as they would not be matched. 

The BNC connector above is also not impedance controlled, so I'd say an SMA would be fine.