Mullions are defined using a Profile-Mullion family template and you can define whatever you want in there. I'd suggest keeping the internal section lines of the extrusion as symbolic lines that are only visible in Fine views (not coarse or medium) and keep your basic outline fairly simple and define that using Model lines - then load it into your project and in your project browser go to Curtain Wall Mullions - there you can duplicate an existing one and select your new profile.
Here we don't bother with the internal profile at all and simply use an indicative profile to provide something of the correct size. At the end of the day it's a very specialist item and we have no way of knowing if it's actually correct - that is why we use specialist sub contractors.
"Give a man a fish and you feed him for a day; teach a man to fish and you feed him for a lifetime."
Maimonides