Of course you can do it!
One way to approach this is how big you imagine your game is.
If one model = one combatant, it makes sense that the army of c. 100 guys comes from a single town/gau/province, and has the same livery.
If one model = 100 combatants, it's reasonable to say that one unit is the muster of one province, and the whole army is a muster from several provinces.
In the end, it's the backstory that binds the army together. I can easily see how an Altdorf general with one or two Reikland units might lead an army gathered from several provinces. The general is dispatched by Karl Franz from Altdorf, and he has mustered as he marched, with an imperial fiat.
Many units in your army can actually be mercenaries from different provinces. It's kind of the Dogs of War, but with Empire units and thus no need for Rare choices.
-Z