As you can see in the images below, I created a construction script for the user to the editor, insert a static mesh.
So it is associated with static mesh the system to see how many elements there it.
Until then, fine.
The second part I created variables that the user can choose extra materials and would store it in the structure blueprint.
I can save these variables in the structure blueprint and then use them during the game?
I think your question is a little unclear, what exactly are you trying to do? From what it sounds like, you want to store data for each material into a struct? You should be able to use an array of structs in order to do this.
What I do is give the user the possibility to choose a SM and then some extra materials. These extra materials are to be used during the game so that the player can change via UMG materials of objects in the scene.