there is no specific button to merge projects but what you can do is to upload relevant geometries in one project.
Also keep in mind that there are some techniques using Blender for instance in order to reduce the file size of your STL files.
I hope that these small hints will help you a bit. Let me know what you think and if SimScale can do something to make the use of the platform more convenient for you.
All the best and happy SimScaling!