I've looked long and hard trying to find something to have the option of giving futa-her balls in sdt.
They'd be hidden almost all of the time, because the girl is in a kneeling position.
If you straighten the girl's legs then balls would potentially be visible, but straight legs tend to look bad (and animate poorly) because the girl's sprites weren't supposed to accommodate standing.
If you want a vanilla import then the functionality would be quite limited. We'd simply draw a
static set of balls near the girl's body, in the same way that we draw claws or wings. The size wouldn't be adjustable and they wouldn't be animated (no inertial movement, no pulsing or bobbing during orgasm). If the modder is thorough then you'd probably get skin-tone adjustment and support for the HSL skin sliders.
If you want animation then this would need to be a
Loader Request. But you should be aware that the probability of completion is low. This project would require a few hours of work at minimum, involving a mixture of AS3 scripting and visual/animation work in Flash. Even if the mod is completely successful, its visual effect would be
barely noticeable. The modder would be forced to
hide the girl's right leg while working on the mod (in order to be able to preview and test it effectively), which suggests that the whole project is a bit silly :)
If not, would it be possible to make it another penis option, like strapon, penis A, and penis B?
Adding options into those lists is a risky proposition, because the related code in SDT.swf uses unguarded enumeration of array elements. If the mod is not implemented very carefully then it can crash the game in an unrecoverable way.
It's generally much safer better to
replace an item within the list rather than trying to add a new one. Or just ignore the list altogether and treat the mod as a typical body-mod import.