I Am Completely Serious

I've never used pumpkin for cats, but it's a classic home remedy for dogs having digestive issues as well, both constipation and diarrhea; along with green beans it's also one of the main go-to additives if you're trying to slim down an overweight dog and need a low-calorie way to bulk out their meals into normal portion sizes.

Does kitty have any food-temperature issues? That's the big thing I'd worry about in freezing pre-mixed gooshy-food-and-pumpkin, since I've known some cats who got fussy even about refrigerated leftovers...well, that, and not wanting to smell warmed up cat food (ewwwwww) if you ever had to defrost stuff in a hurry by microwave -- zapped pumpkin is a much more inoffensive smell.

Either way, if the ice-cube-tray isn't working out well for you here, you could always try doing the divided-freezer-bag trick? Fill a bag with the pumpkin or pumpkin-food mix, not to bursting point, lay it flat on a cutting board, cookie sheet, etc. and smooth out the contents so they're even, then press indentations into it with something like a chopstick, spoon handle, or so forth. Carefully lay it flat to freeze and then you can easily break off individual portions at the thin dividing sections.