In Captivate versions up to version 4, it used to be easy to copy slides from one project to another. by simply importing slides from any other project.
But Captivate 5 lacks the import slides feature! However, another way to copy slides is to use copy/paste. According to the documentation it should be possible to open one Captivate file, select and Copy slides in Filmstrip view and Paste them into another project. I've found though, that in some projects you cannot copy slides. The copy command does nothing in some projects, where in others, it acts as expected.
This problem appears to be linked to the existence of a score page at the end of a quiz.
If you are trying to copy slides from a project that contains question slides with a score slide, try turning off "Show score at end of quiz" and "Allow user to review Quiz" in the Quiz Settings pane of the Preferences window. You can always add a score page back in to the new project if you want.