There is no such tool available as it stands since they are both built very differently.
A .urp from polyscope 5 has no concept of the functions and modules that PSX offers, and likewise PSX doesn’t really have anything directly analagous to subprograms. A .psx file as you’ve seen is based on the JSON format, where a .urp file is based on XML, so they are also completely separate file types too.
The best thing you can do is reuse any script (URScript can still be written and run on PSX) or apply any logic you’ve used to writing a new PSX program, though I realise when it comes to positional data this is a tedious task.