I found that this is a convinient solution.
for example you have the faces and vertices of an isosurface:
[f,v] = isosurface(x,y,z,Gyroid);
and the faces and vertices of an isocaps
[f2,v2,c] = isocaps(x,y,z,Gyroid);
then you can merge the faces and vertices together. Careful on how to merge the faces together, you need to send the vector pointer to the correct position of the vertices, otherwise you will send the pointer to the wrong vertice pointer - for this reason +length(v(:,1)) is added.
f3 = [f ; f2+length(v(:,1))];
v3 = [v ; v2];
face f3 and vertice v3 have the merged geometry!
Now if you want to save the file as stl you can write the following line
If you understant the example I gave, it is pretty straight forward to solve your problem!