There's a few options, most commonly when a mod is integrated into the vanilla game there's a few options, most commonly is probably the dev doing his own version of the mod so there's no issue with copyright or IP shenanigans. I mean, the option to give Lona bigger breasts via body modding at Elise's isn't that far fetched, especially because Elise already offers a host of such services, many much more outlandish than just increasing breast size. The 'only' things that would be needed are adding the option to Elise and creating new sprites for Lona and all the equipment that she can wear. Would increase the work load and I don't know how interested the dev is in this idea in the first place - however it could add some more depth to customizing Lona and make theorycrafting more interesting if boob size came along with some stat changes, like less speed but more sexyness and weakness, for instance.
It's mostly how Rimworld handles adding new content that's pretty close to some mods already existing. Of course you could also hire the modder as a contractor or pay him to have the rights to put the mod into your game more or less as is.
