You'll need to use an Interaction, not a Reaction (reactions are for crafting). You should spend some time reading these pages, they will help you a lot because Interactions are a little complicated (not much different from other raws once you learn how they work, don't be intimidated).
https://dwarffortresswiki.org/index.php/DF2014:Interaction_tokenhttps://dwarffortresswiki.org/index.php/DF2014:SyndromeAt the bottom of this examples page is a prebuilt Interaction for transforming a creature into a demon, you can basically copy it and modify it to your needs, but make sure you understand how to actually use the Interaction on your creature
https://dwarffortresswiki.org/index.php/DF2014:Interaction_examplesYou can look at other interaction examples, in the raw files of the game or on the wiki, for inspiration on other effects. You can do a lot with them depsite the limited amount of tokens you get to use.
To help you out a little, I can say with certainty you are able to do exactly what you want here. You can make a custom creature (giant pig man), allow the giant pig man to use an interaction that applies a syndrome, the syndrome will afflict the target with the CE_BODY_TRANSFORMATION effect and turn him into whatever creature you want, even custom ones.
If you start getting more advanced, you may be able to figure out how to make the pig servants different based on the original creature's race. For example, a dwarf turned into a pig will still retain dwarven features like his short height and beard, this would be accomplished by having different castes of pigmen. However, I'm not sure how I would implement the race check myself, it can be tricky sometimes to figure out exactly what combination of tokens you need to accomplish stuff