Currently only the necessary material inputs/parameters are send to handle all “default” materials: so things like Color, 2 set of texcoord with float interpolant for Atlases (SubUV) when SoftAnimationBlending, float AlphaCursor when AlphaRemap etc… You could hack those existing inputs.
We took a look some time ago to send any PopcornFX Particle Field to any “available” UMaterial input node (like Particle.Size etc…).
But we found some limitations in UE Materials: like knowing which UMaterial node is used to build the vertex stream only then. (well, maybe not limitations, but did not seemed trivial at the time)
But it’s definitively on the TODO list, but no ETA.