I don't know much about the buffalo in general - but I have imported different variants of other aircraft that weren't on the TC type certificate.
If the D model is pretty much identical to the other models than it should not need any other mods except whatever the others needed (eg. fireblocked cargo liners, CDR and FDR instalation or operating limitations) then an application is made and TC adds it to the type certificate. They only thing that may hold you up is the engines and props. If they haven't been certified in Canada you may find it easier to swap engines to a different type. Or again - if they are similar, it could be as easy as an application.
TC will be able to guide you. It also helps immensly if you have a letter from the manufacture of the major components (bombardiar, Hamilton standard, GE) that they have no objections to these parts being run in a civilian fashion, and that they are similar to the models found on the other Buffalos. (On rare occurances manufactures have said in the past that they will not support a product any longer, or support it in a civilian operation - in which case TC won't approve it)
Hope this helps.