It sounds like you are using one project per workspace. But you can have multiple projects in a workspace.
Import existing projects using the menu
File > Import > General > Existing projects into workspace.
The menu Project > Open Project and Close projects are about to close and open projects which are already in the workspace.
This because in Eclipse you should not have too many projects open for performance reasons, see Eclipse Performance Improvement Tip: Close Unused Projects | MCU on Eclipse.
I hope this helps,
Erich