Yes, you can measure any amplifier. (Converters also)
Check the output power of the power amp with a 50 ohm load on the input to be sure you won't damage the NF meter. Measuring the power amp with no input should work. The exception to this would be class C amplifiers which won't turn on without a strong enough signal at the input.
If the gain is high or just to protect the NF meter you can add an attenuator after the amplifier. On the 8970, use 34.4 Special Function, "Enter loss after DUT" to compensate for the attenuator.
Joe thanks for the reply and sorry for the delay. If I'm trying to calculate the kTB power in dBm then do I need to place a filter on either the input or the output of the PA to simulate the operating BW? I'm only operating in a narrow bandwidth compared to its total operating frequency range.