Q: Can I upload my own PBR maps into Poly? A: No, you currently cannot upload your own normal map, displacement map, etc.
No, you currently cannot upload your own normal map, displacement map, etc. However, you can upload your own color map with Upload Image. Mark it as seamless if the image is already seamless. If not, convert it to a seamless version with Make Seamless. Then in the PBR toolbar, select "use render map as color".