You surelly can, but be aware that quite often SMD components detaches of the soldering pads more easily than PTHs, because they do not have PCB board anchoring by leads, and if the crystal is on the opposite side of the board, it would be expected when the assembled board is handled with the side of the solder down (where you intend to weld the crystal), it would be subjected to the weight of the board in addition to the handling forces (e.g repair, extra welding, cleaning, etc ...).
talk to whoever makes your boards and whoever populates them
putting parts on both sides increases cost of board
also need to consider board to board / chassis spacing and mounting
be sure your software handles changing side of board properly