[NiLuJe]

You won't be able to convert Kobo's own dictionary, for obvious reasons (DRM).

But you can deal with the format itself (via dicutil & pyglossary [might need a dev version, but it supports it just fine]). Meaning if you can't find stardict dictionaries, the custom dictionaries thread in the Kobo forum may have something you want buried somewhere in it that you can deal with.