I have never used Desmond, but I would suspect the only way to do it is to increase the area/molecule in your simulation box. This will increase membrane tension. By definition, at equilibrium area/lipid the lateral pressure and tension components cancel each other out. By increasing the size of the simulation box (in one or two dimensions) or, alternatively, by removing lipid molecules (to increase area/lipid) you are essentially applying force to stretch the membrane.