Possible to freeze jam in glass?
Yes, I've done this.  The key is to ensure that the jam isn't filling the glass container completely so that as it freezes it has room to expand.

I used a typical Ball canning jar, which sealed well enough to keep ice crystals out for several months.

From what I understand, freezer canning is useful when you don't have a pressure canner but want to can lower acid items which otherwise might be dangerous to your health (if not pressure canned).