what about routing water in from outside? this is possible now, though there may be z-axis complications later. it might take slightly longer to get started, but its still totally possible.
even still, the idea of an outlying county from which you could muster some soldiers is awesome. also, the idea of seasonal laborers that come and go would be frikkin awesome. imagine once your economy was going, dwarves would come in from outside to earn money by laboring and producing things, then they would leave and take their money with them, excepting that which they paid to rent out rooms and eat.
other dwarves might show up at different times to spend money in shops and things, like the caravan but on a smaller scale and bypassing the trade depot and going directly to shops.
as if that were not bloated enough, nobles might lay down mandates regarding mandatory terms of service, or other interactions with the suburban dwarves.
but i'd like to think of those suburban dwarves as living somewhat like hobbits. they would still burrow into hills and things, but anything that they lacked the underground infrastructure for they could leave on the surface or travel to the fortress for. much like the early seasons of the fortress, when you need to leave a couple workshops and stockpiles setup outside the cliff face.