Hi.
I have this volume created in Houdini and saved to .usd file.
I want to import it to Solaris, but I need to make some adjustments in SOPs first.

I've been struggling a whole day with no success.

What is correct approach in this kind of situation?

You should be able to either:

1- Use a usd file import node and import it onto sops, making sure you unpack into Polygons.

2- If its loaded as a reference or already in your scene graph, use a sopmodify node and again unpack it to polygons so you can tweak it.