Frank,
You can do copy&paste as suggested above, but I think a much cleaner solution in FCP X is to create a compound clip of your entire Project B (select all in the timeline, then right clip and select "Create compound clip". Give it a meaningful name, and then copy&paste that compound clip into the timeline for Project A.
Compound clips are similar to what was called a "nested sequence" in the old FCP. It's a very neat to organize your projects, like folders on your hard drive, and you can go into the compound clip (double click) and make edits there later, without always seeing all the details of it.
- Martin