Different weight cloth is common enough in mainsails. The heavier weight is used where the forces are greater. Different stretch usually isn't an issue because the sail is designed so that the relative forces are the same for each.
Converting a roller furled main into a standard main isn't a good deal. The RF main will be cut very flat, not have battens or a good shape. You will also need to modify/reinforce the luff in addition to attaching slides. You would be spending money
for the main and the conversion and have a poorly shaped sail for your boat.
Better to keep looking for a main designed for the type of mounting/rigging you have.