Yes, I do this all the time. I put fruit/veg in a blender or food processor. One apple goes a long way! Fresh fruit/veg is more chunky and steamed/cooked becomes more of a puree. I put some kibble in the bottom so it doesn't leak out. I also use pureed baby food when it goes on sale.
My usual filling is kibble sample packs, bit of wet food, bit of fruit (blueberry, raspberries, strawberry, apple, banana). I like to use frozen berries since they are easy to have on hand. I also have frozen coconut and toss a couple chunks in the mixture.
Steamed and mashed sweet potato or easy. The XL long fits a full banana!
I have a treat jar, so over time there are lots of crumbs in the bottom. Once it builds up a bit, I dump the crumbs into my kong mixture