[thejeff]

The fortress will boost supplies in it's own province and too a lesser extent in the provinces around it.
That's not a very effective solution though. You want those troops to be out conquering new lands, not holed up in a fort.

So, a couple of suggestions:
Can you use smaller armies? If you've just got 6N on your pretender that's not a really strong bless for Neifelheim, but it's still effective. A dozen or so Neifel giants, blessed and led by a Neifel Jarl should be able to take any indies and all but the largest AI armies. Keep them in cold provinces, it boosts their chill auras and those do much of the damage. Forge some equipment for the Jarl as well. Frostbrand, a shield, a reinvigoration item, Luck pendant if you can. Have him bless himself, cast any useful buffs and attack.

As suggested above: Bags of Wine. hire some cheap indy nature mages, have them site search a bit and then churn out the bags. Give one to each commander or too scouts with your armies.

While you're building that up, avoid waste and other low supply provinces. Plains, farms should have plenty of supply for most armies.