I'd like my immigrant dwarves to arrive with either no labor preferences, or some specific useless labor turned on. The reason is that Dwarf Therapist's "wave number" sorting doesn't always work, but I still want to sort the new dwarves into my community without getting confused about who is new.
It is possible to get new migrants to arrive with no labors turned on (other than the hauling, cleaning and basic medical labors).
In the Init.txt file there is a setting called SET_LABOR_LISTS. Set this to NO will cause new dwarves to not have any labors enabled. They will still have random experience in some labors.
This way you can sort the dwarves by total assigned labors in Dwarf Therapist and all the new dwarves will have 11 labors turned on.
(Source: http://dwarffortresswiki.org/index.php/DF2012:D_init.txt You need to change the Init.txt but that one does not have a decent wiki page yet)
I did not believe this was possible (Though Kevin's answer now seems to imply it is), but i have a roundabout solution:
My suggestion is to set the game to pause when Migrants arrive (defaults to on, also settable in the Data/Init/Announcements.txt), and then, while the game is still paused, read your dwarves in Dwarf Therapist. If you don't have any sorting on, the migrants should show up at the bottom of the list, whereupon you have an easy way to set all of your newly arrived dwarves to the labors of your choice.