It's not possible using the standard Warcraft 3 food system, but you could always create your own system.
You'll need a trigger or triggers that do the following:
Get a unit's Custom Food cost so we know how expensive it is.
Get the player's Custom Food to see if they can afford a unit.
This would tie into the training Events: A unit begins training, A unit cancels training, A unit finishes training
The Custom Food resource would be an Integer array that is tied to each Player using their Player Number.
A unit's Custom Food cost or I should really say a Unit-Type's Custom Food cost could be handled in multiple ways. A Hashtable that links the Unit-Type id to the Cost would make the most sense to me.