Currently Nebulb does not allow direct copying of the sky mask between projects, but there is an indirect way to do it that you may find interesting (which is what I use when I need to do this).
What you can do to make a second project have the same sky masks as another (without having to redefine them by hand) is the following:
1. You have your initial project in which you have already created the mask for the sky.
2. Save this project with another name (this will be your second project).
3. Reset the stacking (in case it is already done).
4. Delete all imported images in the project.
5. Import the images you want to process in this second project.
And by doing this you will see how in the second project the sky mask is already defined and you don’t have to create it again by hand.