[Hoplosternum]

You simply need to add supply to the provinces to stop them starving.

There are st least four ways.

1) If you have actually assigned forces to areas you can assign some leaders too and give them items which provide supply with the #additem command. Like so:

#land 120
#commander "Ivy King"
#additem "Endless Bag of Wine"

2) Assign (enough) nature mages to the province to give the extra supply as each nature point gives some extra supply.

3) Add magic sites with the #knownfeature command that add supply to the province.

4) You can set the size of the population (and therefore it's basic supply value) by map commands too. With the #population command.

Unless you are adding huge armies though simply making sure you are not adding them to very low population provinces should be enough (i.e. don't place your big armies in swamp, desert and mountain provinces which often have populations well below 5000, or even below a thousand at times).