I was looking at your app and it appears to and excellent job of of training for and opening and or a defense.
I see that your system allows input using the FEN system, (This might be a feature request not sure?) my idea is to use the app to store certain positions using FEN and train on those positions.
I will put it in a different way for clarity.
Example: I go to a tournament and I see my first game will be with white (I quickly review my opening moves for white) and then I review several positions (lets say 30 positions) that deal with tactics and or pawn structures in my opening line.
Is this possible with your present setup or does it need to be a feature request?
Yes it is possible. What you could do is mark your 30 positions as "favorite" (long press on chessboard from the "edit opening" screen) and then later train using the spaced repetition training with the "favorite" mode enabled (star button enabled). When a position is marked as "favorite" a star will be displayed at the top right corner of the board in the "edit opening" screen.