You'd need to readd the loading code (and translation table files) in NL, readd the code that converts it for the editor, and on the editor side readd the code that asks NL to do this and loads the file. Alternatively, write all new code for the editor to do the conversion.
Yes, exactly - a
lot of work which I can't really commit to at the moment. But, if someone else wants to volunteer to have a look at it, I will support the effort and help where I can, and merge the feature into SLX in whatever form it takes.
What would the purpose be? You've said you aren't worried about existing content
Converting older levels would be one motivation, but also having a cross-platform Editor that's up to date is a tempting prospect.
When I say I'm "not worried about existing content", I mean it more in the context of adding new features which may render existing content unplayable for whatever reason; if the feature is desirable enough, we shouldn't refrain from adding it for the sake of levels that were made for a different platform (and are still playable on that platform).
Otherwise, previous content is more than welcome on SLX, and if we can have an Editor that makes converting older levels easier/more direct, then it's a worthwhile pursuit.
With that said, I simply don't have time to look at this myself at the moment. I can only offer help and support to anyone who might want to take the project on themselves.