[NTG-context] Defining and using font weights

Hello, I'm not able to figure out how I can make use of a font with "custom" font weights. I have a font that comes with "regular", "bold", "half-bold" and "ultra bold". I currently define the font like this: \definefontfamily[myfont][rm][myfont][sc=file:MyFontRegular,bf=file:MyFontBold] Which at least makes it possible to use the default font and \bf ... But simply adding something like hb=... or ult=... doesn't seem to cut it. So how would I define this font to be able to also use the half-bold and ultra weights? (preferably with a switch like \bf, \it, etc.) Best regards, Andreas