I mainly use Crazys Mod, so the 0.7.1 editor might not have the option, but it should be possible to either
- add a chance for the trait "is daughter" to a random girl
- add the "is daughter" trait directly
If the option is not there in the editor, you could edit the text files yourself by comparing and looking up how it is done by the 5 daughters already in game.
And of course there is the obvious way ingame, which works in the mod at least (can't say about the 2014 version of the original game, but i think it could create daughters already, too).
Also compatible girlpaks might have them.