My experience has been that conversions are cool. As long as there is Excelsior product in there, then feel free to do your thing.
My Bloodbone Axemen all have Rackham heads, as does my Mauler. I've played against Thom, and also faced JP at the Southern Exposure tournament, so I'm going by what they've said to me. You can proxy any model that isn't produced yet, and even some that are, to add flavor to your army. Bill Refsland ran the last tourney and had a wild assortment to work with for his Sons of Kronos, including different packs of war dogs to differentiate between them. So with that, go wild.
Post pics of your work please!