analog signal generator output in Volts vs dBm
I guess if I set the MW source output to 10mV, given the load
  impedance of 50 Ohm, it's power output is 500mW?


Power is \$V^2/R\$ so 10 mV into 50 ohms is a power of 2 micro watt or 0.002 mW or -26.99 dBm. The final conversion takes the log\$_{10}\$ of 0.002 then multiplies by 10 to produce dBm.