No, merging/splitting of projects is very complex due to relational data (e.g. comments, likes, views) and we do not support it. If you wish to do these kinds of operations, you would need to re-upload assets to the relevant project.
A project is a distinct artwork. Some artists make the mistake of dumping a whole bunch of different works into a single project, which appears as a single square on the main ArtStation walls. By posting more artworks, it gives more of your work the chance to trend, appear on people’s activity feeds and to be shared on social media. It also makes your profile look more impressive with a larger body of work.
Don't do this.