Hey Lemarchal , thanks a lot for your inquiry. Pardon, I'm having a bit of trouble understanding your request, I'll give you an answer and if this shouldn't help, please be so very kind as to briefly rephrase what you'd like to achieve. 
With A-Mask I suppose you mean alpha mask? If so, you can click the checkbox "Transparency" in the Enscape Materials window and load in a alpha mask accordingly (by clicking the + right next to "Texture").
Furthermore, and I'm not entirely sure if I understand it correctly, but you generally want to have some reverse bump in the gap, but not on the other surfaces? If so, please be so very kind as to send me the textures at hand and I'll assist you further with this!
Thank you in advance!
