You can rebuild the app, it’s built with Electron. The WXR importer code is easy to read. All it needs to do is to read a field from your WXR/XML file. I manually edited the slugs but I had 43 posts, not 400, to edit.
That the slugs don’t allow slashes is annoying since it means you need to set up redirection if you’ve used a path pattern such as/topic/year/title – which is common. I solved it by replacing slashes with underscores, used that as a slug and then set up a rewrite rule to redirect traffic to the old paths.