Is it possible to craft attachments, and if yes, then where can I find the rules?

It would be logical that some attachments are not craftable (or at least, not unless one has access to an industrial plant and/or some really rare raw materials).

Lightsabre crystals should probably not be craftable, as they come from natural formations/animals ... on the other hand, that statement gives fresh support for why/how the Sith - in Legends canon at least - made artificial lightsabre crystals. (I can't remember if the process automatically made them red, or it was a question of the ingredients used, or it was simply a political statement.)

There are no official rules to craft Attachments, but you could extrapolate from the crafting rules in Special Modifications.

A good rule of thumb would be using half the cost of the Attachment and assign a Difficulty based on cost (I'd start with PPP) using the other crafting as a guide.

I've actually homebrewed some rules for it. I'd really appreciate feedback! SWRPG Crafting Spreadsheet

Somebody else had a topic for a fan-made supplement recently for crafting lightsaber crystals. It probably isn't too many pages back, but you could just search for it.

From my limited experience, treat it as crafting a Mod . Thus the PC would need to gather unspecified parts and then roll a Mechanic check to put it all together. For the difficulty, you have to use common sense. A tele-sight(or whatever) would be much harder to make than a bipod. Example: Bill wants to make a tele-sight. The GM rules that first he must find the parts(shops, junk pile, etc.). Then the GM makes him roll a daunting(4 purp) roll. Use the same rules for modding attachments(on die results).

Jedi may not craft attachments.