I'm just starting GMing a game and I have a player who wants to mod the crap out of EVERYTHING. For example: He found a pair of weapon detection goggles in a source book. He want to mod it so it can detect weapons, thermal vision, and night vision. Can you mod anything in the game or just things with hard points?

Anything other than improving attachments with mods in specific ways, improving weapons/armor in specific ways with jury rigging, and making items using construction rules, is purely up to GM.

You mod attachments to items, not items, with the exception of Jury Rigging and Tinkerer, which are pretty narrow in what they do.

I think that for what he wants to do he should get an armour and then add the General Purpose Scanner attachment and the Enhanced Optics Suite attachment. Narratively you can say that the optics go into a pair of goggles if the armour doesn't have a helmet (or the player doesn't want to wear one), so long as you use the appropriate number of hard points from the armour itself.

In this game, attachments and modifications are pretty simple in terms of their game mechanics, but how they are flavored by the GM and/or player is basically limited only to your imagination.

So, if the player wants to mod a pair of goggles to do something extra, you could let them do that as a pure flavor thing — nothing mechanically changes about the goggles. Or, maybe there is an actual mechanical change, in the form of a boost die or removal of setback dice, or something else as appropriate. But none of these modifications at this level would use hard points or other more concrete mechanisms, it’s just between the GM and the player(s).

Once you get to armor or weapons (or vehicles) that do have hard points, then you can go with standard existing attachments and modifications to those attachments, or you might want/need to make up some of your own.

The flavor and fluff of those attachments and modifications is entirely up to the GM, and that’s separate from the in-game mechanics of how that functionality is recorded on paper.

To take an example from my own experience, in a game a while back I was playing an LE-series Repair Droid (like “Leebo”, see http://starwars.wikia.com/wiki/LE-BO2D9). As the game went on, my character got more and more into self-modification, and one of the things he did was to occasionally take goggles or other types of optical gear and incorporate parts of them into his own chassis.

I figured that the horizontal and vertical strips on his head were actually channels that the optical sensors would travel on, and that he could make additional optical sensors to fit into those channels. So, take that picture you see on the page linked above, and imagine a few more optical sensors mounted on the helmet.

My GM was fine with this process, so long as I paid the money or otherwise appropriately acquired the optical equipment in question, and did the mechanics roll to incorporate them into his own chassis. And so he got the full benefit of having all those various types of goggles built-in, but he would have to move them around on his head to connect them to the circuitry for processing optical sensor data.