I suppose it isnt possible to add open-close eyes to a doll that does not have them?
It’s possible I’d image, but how difficult would depend a lot of the doll you were using as base (esp how big the doll is too)
If they already have fully round inset eyes, specifically the kind you inset through the front that have plastic behind them to hold them in place, it would be a matter of getting the right size eyes with the mechanism (the easiest size to get being for ~18″ dolls):
Other wise, you run into the issue of getting them in and to stay in place.
You can def cut open the back of any doll’s head and carve out the molded eye shape for inset eyes, and then just glue them in place and reattach the top of the head, so I imagine it’s a very similar process, just be super careful that you’re not gumming up the mechanism with the glue.
Before you do anything though, I’d say it would be a very good idea to make sure the size of eyes you want are available with sleep mechanics.