1: First you will make the mesh.
2: Then make each part that needs its own texture a separate object.
3: Export as OBJ. Minetest does not use materials and disabling it will save a bit or two. Check "Objects as OBJ Groups" and uncheck "Objects as OBJ Objects". I don't know why but it is needed for multi texture support.
If you do not need texture syntax compatibility you can stop now.
4: Rename top to f, bottom to e, right to d, left to c, back to b, and front to a.
5: Export as DAE. Check "Sort by Object name", it is needed for texture syntax compatibility. Watch out for "Triangulate" but you don't really need to.
6: Import the .dae and export it as OBJ following step 3 again.
Then you can use it in minetest.
P.S. If you are a maintainer of moreblocks you should look at the pull requests.