Is it possible to make a femboy model in this game using the male body?
Yes.
Just try the slider options, they're there precisely for that (duh!).
You can use the basic image-sliders (the ones that have a picture of types of bodies), and then can fine-tune details switching to the advanced mode.
Note:
If you make it too feminine, there's going to be some minor clipping thru clothes, as male clothes aren't fully supportive if you make a big round ass guy or give them tits (you can't give them real tits, but you can achieve a sort of A-cup with chest and nipple sliders), and no matter how feminine you look, you can't use female clothes.
The same way, you can "masculinize" a female/futa.
In this case it works better visually, but inverse to the previous example, you can't use male clothes.
This way has more physical options. For example, setting low attractiveness doesn't means being uglier, it's just that features are less delicate and less make-up, which means you can look more "tomboyish".
It also has sliders for more androgynous bodies, and more penis options (in case you want a really small cock, or a huge schlong)
It depends on what you specifically want.
Why Vin doesn't uses the female model to make males
and females/futas is beyond me.
The female body model has a much more varied range of sliders, while the male is very limited.
It will reduce the game's size, solve several issues (like more varied dicks, everyone could use all clothes, and other things), and overall simplify things.