You can't convert Marvelous Designer cloth to quads. 

@Nargool well, maybe if you have a very old version of MD.

In MD5, 6, 7 and above, you can convert all clothes meshes from tris to quads and export the Marvelous Designer quad mesh.

By default, Marvelous Designer clothes have a tri mesh. To convert tris to quads,  select all the garments or clothing parts in your scene which you wish to convert (CTRL+a to select all), then right-click on any one of the selected clothing patterns and select "Quadrangulate" from the right-click menu.

A progress bar will appear (the lower your particle distance, the longer it will take for your Marvelous Designer tri mesh to convert to quads. If your PD is VERY low, it might even crash the program).
Once it's done, you'll see your mesh change from tris to quads (to see the mesh, turn on the mesh view mode).

To view the Marvelous Designer quad mesh, click the mesh icon here:

@camille-kleinman cool, I didn't know that! And how do you export the quad mesh? Just by regular OBJ export or do you have to set something in the export settings?

If you export the clothes after converting them to quads, they'll export as a quad mesh from Marvelous Designer.