It is possible to change Bonehilda appearance in Cas permanently?
I edited Bonehilda in cas, giving her a human appearance. When she return to the coffin everything is fine, but when she exit again is in her skeletal form.
My objective was making an "army of nine hildas" in human form to keep clean the villa (is very big, 20 sims living in) because it seems a single maid isn't enough and i don't want to use a butler.
I even built a nice crypt for their coffins, well hid below the garden: i hope it's not time wasted

Is it possible to block the coffin so Bonehilda never has to enter it again?

If it won't let you block the coffin normally, you could also try using the "moveobjects on" cheat. I've used it to put gnomes where I didn't want sims to go.

Good advice: i will try to block the coffins and see what happens.

I tried to block them destroying stairs to the basement crypt, but they teleported into coffins, i built a fence but they teleported again.
So i tried another thing: i deleted their coffins while they were out using "moveobjects", then i warped a pair of days to see what happens: guess what? It worked!
Now i have 10 human bonehildas working day and night :D

Ah, nice to see you figure out the workaround. Perhaps Bonehilda doesn't exactly need her coffin after all.