If you can get them on embark you can get them through trade. You get peasants.
I mean to see if they are still gettable from trade caravans once I mod the raws back to normal after worldgen. having the PET tag screws the dwarves up during play, because you can't access their labour choices (but they still seem to carry on with assigned labours, you just can't change assignments until the game is loaded without the tag). Dwarf Therapist or a similar program could probably change assignments with the PET tag in place, but i'm using the latest DF and Therapist is a couple versions behind.
I agree extra peasants sucks from a game-play point of view, which is why I didn't test this completely before. Extra at the start would help a lot more. Maybe have the extras be wood choppers, military trainees and haulers, etc.
EDIT : Embarked with 7 'extra' dwarves. Noted what was so off-putting the first time. Each extra dwarf is listed as e.g. "Momuz Mololok Stray Peasant (Tame)". All dwarves currently get the same interface as a pet with 's' for ready to slaughter. Running slaughter test now with butcher, tanner, leatherworker ...
Ok slaughter worked easily enough with dwarven body parts now cluttering the butcher. But no tanning job is generated at the tannery, so no dwarf leather
And built a food stockpile but the dwarves aren't stockpiling the dwarf guts etc
. Ok they will stockpile the dead dwarf sushi in a 'corpse' stockpile. This may be changeable in the entity_default.txt dwarf civ settings.
EDIT2 : I'm now looking at the animal screen, the extra dwarves are listed here, and can be set for adoption or slaughter. I brought an animal trainer and genned the world with dwarves having [TRAINABLE]. Time for wardwarves training test.
FAIL. No luck. I didn't have the tag after all in the save, Silly me. The 'pet' dwarves ARE war-trainable. I now have "stray war peasant (tame)". Now for adoption test. Reloaded with the PET tag gone (couldn't access dogs / labour on any dwarf with the tag there), they are no longer 'stray's but are 'war peasant (tame)' and am able to assign war dwarves as pets to other dwarves!! They still do jobs, but cluster with their 'owner' when not busy. Renamed custom profession on one to 'Slave Girl (tame)'. Hmm after training some of the tame peasants, can't seem to re-allocate labours on the trained ones, even after restoring the raw (though they are still doing jobs as normal), and they still appear in the animal section, are no longer listed as part of the pop limit, and can't be added to a squad. Still, war training should make them a bit tougher, they might make good general hauler/mason/mechanic types, with woodcutter skill so they grab an axe too.
Epilogue : Dwarves are still listed in trade goods of my civ even though pet tags are gone, i've requested male and female dwarves at max priority. Also of note, you can delete the extra tags before embarking, edit the region folder straight after creating the world. I was able to buy hunting and war dwarves at embark with [PET][COMMON_DOMESTIC][TRAINABLE] during worldgen., though they are a little buggy. Dwarf therapist should fix it completely. Final epilogue : traders delivered 8 dwarf cages, and they are actually spawned with existing professions, unlike those bought on embark. Now I can't seem to get them out of the cages! Linking to a lever. All the cage dwarves were @ symbols rather than normal citizens, those that were let out ran off the map edge straight away
. Some turned into normal face symbols after a while, for no apparent reason, while still caged, and stayed on release.