I have been told that once used, it is not possible to Kasher a pot with a glass lid. If that is true, would it be possible to immerse them new, right out of the box and use them?

Used glass lids (and pots) can be Kashered You should unscrew the knob and thoroughly clean the lid (or pot) from any food remanent. Wait 24 hours since last used hot, then boil water in a clean pot that wasn’t used for 24 hours and immerse the lid when the water is bubbling so the water covers the lid totally (even if not in one go). There are alternative ways, this is the easiest. For a new (metal or glass) pot – if made by non-Jews you must recite the Brachah and immerse in a Mikveh (or the ocean) before using.