Can I alter rubber boots?

Here is a cute idea that may work: https://www.practicallyfunctional.com/diy-lace-rain-boots/?utm_medium=Social&utm_source=Pinterest

Using the above idea, you might be able to cut the back or front of the boots down a few inches and insert a stretchy fabric piece which could be glued to either side of the boot top (sliced area) to provide some additional room.

Could you slit them down the back and maybe add gromets and a ribbon to make it adjustable?

Thick elastic sew inbetween slits cut down the sides

I am new here, did you try to turn/fold down the tops of the wellies ? this should help to get them up !!

You might try snipping them about 3 or 4 inches with a pair of scissors right down the middle of the back of the calf. Then use some strong glue, like crazy glue or gorilla glue, and attach Velcro strips so that you can fasten them with the Velcro straps after you’ve put them on.