You can't in MTW2, war is initiated by an attack. You can however declare 'do X or we will declare war', which I think gives you less of a reputation hit, when you then do declare war.
I have a role-playing house rule as the Romans, that I always try my best to return generals to Rome after their imperium expires, which is six years for either praetors or consuls. I also only lead major offensive campaigns or sieges with generals who have imperium and were elected to office for that purpose, although I make a few exceptions for younger guys in their early 20s by having them lead in support roles, using them to damage or wipe out enemy field armies around enemy settlements before besieging them with the main army led by an older, more capable general. It's also just wise to start a general's military career early on so that they build better command traits and gain more stars and confidence points from the get-go.
I love that, youngsters with starter armies clearing out the way for oldsters with the hammer.
In my Mak campaign, I send every come-of-age FM to Sparta for training in the Agoge and a couple years of Paidiskos work for starters. Once they hit 20 and have that under their belt, I send them all throughout the vast empire to put down minor rebellions and slave revolts and whatnot to further their training. That usually takes about another 2-4 years by the time they return back to the capital as veterans, having built up experience by fighting smaller, manageable battles.
They cover way more ground without attached armies of course, so they travel to the hotspots alone, and requisition armies from the empire's client rulers in the area, as they are obligated to provide when called for. Then those units head back home and the young general moves on towards the next band of freedom lovers to bring them the freedom of death and slavery.
or use them not as generals, but just as a unit in your client general's army, and have him travel around to different battlefields.... You know, like a JEDI for the old republic... Although he would not gain leadership experience from it, it probably would give him some hit points retinue/traits...
I like the idea of doing stuff like that, maybe moreso as Rome where that's how they're meant to be used at certain points of career I think, but like you said the lack of command experience. Also, in my case, the generals I'm sending up are part of the Makedonian elite and ruling family, no way they're playing second fiddle to some barbarian warlord who's only being kept in meade and honey by our divine whims. (I pretty much have an evil slave empire thing going with most of my dudes being elitist douchebags ruling with iron fists, it's great)
wrong thread, sorry