If there is a door or opening I'm sure they can get in. To send a settler to other towns as a provisions you need 1 point in local leader perk (under charisma). Then you in build mode you go up to a settler, select them, press a button (I think it's l1 or r1 on the ps4, you can see at the bottom of the screen for the button action that says trade route), then the list of all your settlements shows up and you just select one. That settler will start going between the settlement it originates from and the one you picked and the workbench junk between those two pages will be shared