In Glyphs 3, you create a new layer, right click it, and choose “Intermediate master” as the layer type. Then you set up the axis locations through a UI, in contrast to Glyphs 2, where it relies strictly on the name of the layer being in brackets and having the right values
oh and about your second question: if you want to interpolate the whole style, then why define it as a master in the first place? you can export it as an instance without needing a master for it if it’s 100% interpolation.
if you want to make a new master anyway, one method would be creating an instance at the axis location (weight) you desire, in between masters, and then you can add that instance as a master