It's easier to work in voltage.
Assume your sig-gen is an ideal sinewave source in series with an internal 50 ohms.
Connect the gen to an external 50 ohm load, which it's advertised to dump 1 mw into. So the voltage across the load is 0.2236 volts RMS (which makes 1 mW into 50 r)
So we have a generator feeding a 50:50 ohm resistive voltage divider giving us 0.2236 out. So the ideal generator is making 0.4472 volts inside.
Now just treat situations like you propose as a simple voltage divider, do the ordinary stuff, and once you know the voltages you can convert any element's voltage back to power and dBm if you like.0.4472v--------Rint50--------+-----------Vout | | | Rload | | gnd
so it's just like a DC voltage divider.