How can I support drywall with a steel post on an outside corner?
In light of the new and important information mentioned in the comment below,  your best move is to fur out the entire wall just enough to clear the post. You may then want to follow the advice I gave originally.



Original answer:

You can work with that and avoid any sort of bumpout. Just cut a triangular "stud" as large as possible that fits in the available space. Set your circular or table saw to 45 degrees and cut a few sample blocks. Use a straightedge across each wall plane to see what you have room for. You could even hang drywall on one face and then install the stud with screws through the drywall before you add construction adhesive to all contact points.

Ideally you anchor it top and bottom, and even a gob of construction adhesive against the post flanges will do. Even better if you can glue to the drywall and the post itself. As a system it'll be rock solid. Let the adhesive set up for a day before nailing on your corner bead.

If you're wrapping that beam with drywall, install a strip of half inch plywood or OSB under the beam, between the flanges at each end, to get the drywall below the flange and bolt heads.