Only if they are all diplomats or infiltrators... 
Because it's so hard to find/become a diplomat.
Unless you were talking about raising a few hundred men, then the forces you are raising would be pretty ineffective. And if you wanted to raise a decent sized force, the costs involved would be absolutely huge. For comparison's sake, take the cost of men at an average RC, of perhaps 40 gold/10 men. To raise 100 soldiers would be 400 gold. And you'd probably need at least 200 to be of any real use. So that's 800 gold just to hire them in the first place. And then you'd have to pay them at a rate of 330 gold a week. And of course you'd have to pay them, having them pillage and maraud until they were killed would be a bit ridiculous. Your average mercenary wouldn't do what you want without payment.
So we have a cost of 1,130 gold for the hiring of 200 front-line quality soldiers and their equipment, for one week. That's a lot of gold. If you want them to last for two weeks, that's 1,460 gold.
But where are these soldiers coming from? Do we really want to assume that soldiers of this quality training and equipment are just hanging out in the middle of nowhere waiting to be hired by some shady noble?
I don't think that this suggestion really makes much sense, unless the quality/number of troops involved were kept so low that they really wouldn't even be worth bothering with. Otherwise the costs involved would be so prohibitive that only the wealthiest of realms could possibly afford to invest in them. And by that time you'd almost certainly be better off just investing in your own army to begin with.
