GW's view from the
TOS rules:
"Miniatures
• All of your models must be fully painted and based and fully represent what is on your army list (including all
equipment).
• Every miniature and component must be produced by Games Workshop (Citadel, Forge World or Warhammer Forge). • We know many people like to take the opportunity to convert models for thematic and creative reasons. We actively
encourage this, but do ask that if you are going to do so, please check with the events team first. We may ask you to make allowances at the event in order to ensure there is no confusion for your opponents."
http://www.games-workshop.com/MEDIA_CustomProductCatalog/m3010062a_Throne_of_Skulls_Rules2013.pdf
So, ye,
GW are fine with it. I don't see why anyone else would have a problem.