Is it possible to make a jam-setting gelling agent at home

From the National Center for Home Food Preservation:
Making Jelly without Added Pectin
Making Jam without Added Pectin

• Use a mixture of 3/4 ripe and 1/4 under-ripe high-pectin fruits. Under-ripe or just barely ripe fruit contains the most pectin.
• Cook the fruit with cores and peels to add extra pectin (but do remove stems or pits). Put through a sieve before adding sugar and spices.
• Citrus peel contains lots of pectin, so consider adding some of it to your mixture.

Fruits low in pectin: apricots, blueberries, cherries, peaches, pears, raspberries, and strawberries. That's not saying you can't make jam/jelly/butter from these without added pectin. It just may be a little more difficult than, say, using apples. In fact, as an example, here's a pear butter recipe w/o pectin.

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).