How do you get a model that's shaded with Renderman shaders into Houdini with the best approximation for the shaders in Mantra. Is that even possible or do you have to start from scratch.

I tried exporting it to fbx and it comes into H just fine but obviously the shaders look off.

I honestly have no idea, but I believe you have to use PRman shaders. I could be wrong though; I think there might be some pages in the docs, these are the only two I could find:

https://www.sidefx.com/docs/houdini14.0/nodes/out/rib [sidefx.com]
http://www.sidefx.com/docs/houdini14.0/render/renderman [sidefx.com]