It would be great to mix in after you open the jar. Generally you cannot use oil in canning, so I think jam with peanut butter in it might be susceptible to botulism. Below is a link to a thread about canning peanut butter. Somehow peanut butter is shelf-stable, but canned products with oil are not.

Peanut butter and banana jam sandwich, that sounds good.

Melissa

Here is a link that might be useful: canning peanut butter

Commercial peanut butters do have preservatives, one of which is a bit of vitamin E. They do go rancid and can act like oils when using in home canning. And as you know oils in home canning are not advised. Better to mix afterwards

I'm not sure it's just the oils, either. I think the peanut butter may have some density issues when used for canning.

I would question the acidity of such a mixture, and if it's thick as I envision, the density could prevent an evenly heated processing.

Annie