Is there any way of adding Magma to Pflow?
Can this be done indirectly with a non-PRT Save & Load workflow?
No.
It is on the Wishlist, via a planned PRT PFlow object which could turn a PFlow into a PRT object without intermediate saving.
That object never made it into the KMX 2 development schedule. 
It is a sad fact that it can’t be done like in the old channal modifier, i hope they add it soon.
With that said, there is a little trick to work-around the save/load workflow.
In you Render Elements add a Krakatoa CustomData element. In the settings for this element you will get a magmaflow.
Now when you render you will get a seperate render with the desired effect directly from your Pflow
hope this helps