As purchased, the 635 is not waterproof. Applying silicone around the case would be feasible. The one difficult area would be around the buttons. There would be a thin line between making this area waterproof and still have the buttons function.
The silicon rubber the buttons are made from would work well as a water barrier. What I would do is on the inside of the case, build up a mound of silicon around the perimeter of the button opening then when the silicon cures, use a razor blade to trim the top of the mound flat and level then when you assemble the unit, the silicon mound acts as a seal under pressure that will be in contact with the edge of the buttons silicon membrane and the PCB. If you do it right (and can understand what it was I just said) you should have a good water resistant seal and still be able to use the buttons.
The best that you would be able to do with the XES635 series would be "water resistant". The case itself is a clam shell steel case that is "not" sealed. Along with the idea that Heffo suggested for the keypad, you are going to need to seal around the case seam, the cable opening and around the LCD and LED openings as well. It will take some work, but with care and patience, you should be able to seal it up rather well.