Possible yes, I would argue it probably wouldn't even be all the difficult, tedious maybe but not exceptionally difficult.
as it is right now you can give a male character boobs with majorus potions (or cheating by editing the save file), though that is a fairly surface difference, and requires you to fill in the gaps, as far as the narrative goes.
I imagine you mean having them randomly generate, and actually be referenced as such in game.
in that case aesthetically you would just need to decide on what to call it, what the gender pronouns for the dialogue replacers would be, and give them a gender icon. I think setting all that up would be reasonably simple, but I don't know where everything that needs to be added lives.
As for mechanically you need to add it to the character generation "if/else" trees for breasts so they generate larger than masculine, or from the mother rather than the father in the case of breeding (fairly simple change could actually streamline that part of the code the way I'm thinking it would look).
And the automatic reassignment from male to shemale when adding breasts to the character or changing from futa to shemale when changing the breasts or genitalia respectively (probably not too difficult either but that raises the question of is a 'male' with any size of breasts over masculine shemale or do you set a size threshold, this does complicate existing things slightly but not overly so).
You would probably also want to set a rule toggle like exists for futa, or futa with balls, for those who don't necessarily want shemales to generate in their game, if you were to implement it for more than personal use. This one is not something I know how to do outside of the "editing the scripts directly" method.