Think impedance! anything is possible but be more clear for example a newer taylor guitar with a expression system with its designer pre-amp is designed to do just that.. go out xlr as well a early Studio gibson les paul but some guitars pick-ups well only cause you frustration perhaps with a little research there is a product by the same company that address the impedance issue

Welcome to RO, munkee:
Where would the home recording world be without the POD, eh?
The XLR input on the POD X3 was designed to be used to process a microphone, not a guitar. If you're wanting to do THAT, fine. As previously stated, there are impedance issues, as well as LEVEL issues, that are going to be a concern plugging another guitar into the box.These will not damage the unit, but it will sound like crap.
The adapter you listed isn't going to help this. Yes, you could run a second guitar (or a bass or a keyboard, etc.) through a DI box to adapt it electrically to the XLR input. Not knowing the application you had in mind, it is difficult to say whether it will give you the sound you're after.
We can suggest some DI options if you'll supply us with some more specifics on what you want to plug in...

Thanks for the tips! All I want to do is add a second guitar for a friend to play along with. No big deal. Not even for recording. It would be an electric guitar. It does have a trim knob if there are level issues.

A "standard" electic guitar will require a DI box.

What one would you recommend? :D

An "active" DI would probably be best for guitar or bass. Whirlwind, Radial, and Countryman are the better ones out there. Frankly, I don't care for the sound of a DI'd guitar if a "natural" overdriven sound is what you want, and I have no idea as to what processing the X3 offers for the XLR input. I doubt that it's designed for electric guitar. You need to investigate this before investing $50-200 in a DI for this application.