You can use it to make members of a specific species legitimate members of your population, but there are a few problems to consider first:
- When you make them members of your populace, they will immediately realize they are naked and freak the fuck out. Make them all clothiers for a while and have them produce tons of clothing - if they are not the same size as your fortress members, they will need to do it themselves. I had dingo people, and they were too small for dwarven clothes, but would produce clothes appropriate for themselves if given the task.
- Previously-wild or otherwise foreign individuals will not be properly recognized as members of the population, and thus you cannot assign them to noble or military positions through normal means.
- Members born in your fortress, especially those born of pregnancies started before their mothers were made members of the civ, will be able to be given military positions in some cases. Their relationships screen won't necessarily show their mothers or siblings, and it will never show their fathers, but regardless, if one of them dies their family will be devastated.
- They may be slow to make relationships with one-another, and thus may not form any married couples. I never saw a romance start between any of my dingo people, but that could just as well be due to a shortage of variation in the families. They won't marry their siblings or cousins, after all.
- Because they might have trouble forming relationships, there may never again be new pregnancies within your population, and it could stagnate and eventually decline.
- I had to invoke the friendship command every time I loaded the game or they would forget what they were supposed to be doing and cancel their jobs to go be tame
I have no idea whether the peoples you made members of the fortress can arrive as migrants in later forts.