Can you add the "Airtight seal" enchantment as a modifier?
Also the ability to attach a pressure tank on chestpieces?

Could you let me know if this is possible or not? Because In my modpack you can't use any armor unless it is for the player's temperature or Construct Armor.

How about the airtight seal enchantment?

It would still be nice if you could do it with the tinkers table, dont get me wrong.

Technically you can already do this, advanced rocketry has a recipe which adds this to any armor piece, even Construct Armor. just the JEI recipe looks broken.

Lemme get this straight, all that's needed atm is a variant of the Airtight Seal enchantment that works as a modifier for Armor parts?

Ye

Is it only valid for chest pieces?

no