If I may ask since I have never actually attempted to do this, how do you actually summon? I tried to research this, but it was very sparse from my search.
That, like many things in Dwarf Fortress modding, is also fairly roundabout. There's a couple methods, both of which share the same basic concept: You define a material which will give your Amazon a particular syndrome upon ingestion, and that syndrome gives the Amazon access to the interaction that she uses to summon more Amazons.
The first method, which I'm more familiar with, is this: You define a new material with a very low boiling point, such that it will immediately dissolve into a gas at room temperature. You also need to give this material a syndrome that will be applied to your Amazons when they inhale this gas, which will give them access to an interaction which allows them to summon creatures. You then need to make a reaction (effectively a crafting recipe) to create a boulder (any type of item probably works, but it's typically defined as an inorganic material) made of this material, and make that reaction accessible through a workshop. You assign an Amazon to create this boulder, the boulder turns into gas, the assigned Amazon will inhale the gas, and then she should hopefully summon the new member(s) of your fort. For lore, you could think of it like using a special blend of herbs which they mix into an incense and inhale the fumes. I unfortunately lost the files for the mod I made, otherwise I'd be able to give examples of how I did it.
The second method is similar, but the material is instead defined as a food which gives the syndrome upon being eaten. I'm given to understand this is more consistent than the vapor method (because sometimes they just don't breathe the vapor in), but is also slower and gives less control over who does the summoning, because you can't micromanage what or when your citizens eat. However, I was able to find some example raws in
this thread, supplied by Eric Blank. I recommend looking through these no matter what method you go with, as it includes the most important part: The actual interaction used to summon the creature.
There is also a much, much simpler method, which would be to make Amazons innately capable of summoning other Amazons, no crafting or workshops necessary. This has its own problems, because populations are likely to balloon out of control if citizens are allowed to summon more citizens whenever they feel like it. You can limit the allowed frequency of a given interaction by using more syndromes, but as always, that's pretty complicated.
Hope this helps! I wish I could give more specific examples, but as I said I lost my existing files and it really is a lot to rewrite off the top of my head. Let me know if you have any more specific questions!