No, you can not just put an attenuator at the output and measure impedance. The network analyzer does not really measure impedance, but instead measures something called reflecton coefficient. So, if you put a 20 dB attenuator at the output, your analyzer sends out a signal, that signal goes thru the attenuator and gets attenuated by 20 db, hits the amplifier output port and has perhaps a 5 db return loss, goes thru the attenuator AGAIN, and loses another 20 db. So the returning signal to the network analyzer is 20-5-20=-45 dB. A 45 dB return loss will look like exactly 50 ohms every time, no matter what the amplifier is doing!
You could connect the amplifier to the load with two directonal couplers in between. The couplers are installed in opposite directions. Then you put a signal on the input of the amplifier, and you use a power meter on the two coupled ports at the output. There will be a "standing wave" on the line between the load and the amplifier output. The power meter can measure forward travelling power, and reverse travelling power, from the two directional couplers. So you can easily figure out the MAGNITUDE of the mismatch between load and source.
If you actually want to know the source impedance, you need to do a "load pull" measurement. There are various ways, and all are complicated. Do a search.