Hi Nikolas,
I supppose you're talking about HaRe, that Thomas Schilling linked to. I have no idea how that system is built so I can't answer your question. But in principle I don't see why not. :-)
In principle it would actually be quite difficult. HaRe is Haskell 98, built upon the Programatica front end, and to use extensions would mean a port to a system that has extensions (ghc say) and a re-evaluation of all the current refactorings so that they work over extensions as well. Porting to a system like GHC would require an almost complete re-engineering of HaRe, and, although simple in theory, in practice it has so far presented some problems. With the exposure of the GHC API though, we hope to address this issue in the near future. Kind regards, Chris.