Hi @cevasco.biz and welcome to the forum! 
Currently it is not possible to do that on the Mini without an external controller.
Which is why the community has come up with two wishes for this:
Someone is also currently developing an add-on, that will attach to the bottom of the Mini.
Good question, i guess the main reason would be that it would not be possible to traverse a music scale quickly enough, so that using +/- would make sense. The composition process on the Mini requires definitely a lot more planning if you don’t have a external controller.
The other guess is that the controls are probably heavily inspired by the Gameboy/LSDJ, which also doesn’t offer live recording directly on the device.
I think the wishes that have been created by the community are a very good balance, that could bring that functionality to the Mini at some point (hopefully). 