There has been some discussion on making user dictionaries in the
Kobo Developers subforum. Unfortunately it's very much a do it your self project.
A prc file is a container, so you would have to get the dictionary out of it, and into a format that the tools for making a kobo dictionary expect.
And yes, while calibre can convert between many formts, it tends to break when converting files with many internal links, like dictionaries.