We're talking different things here. There are several ways to add a project's files to an existing project.
What was suggested above is to simply break your very source-heavy effort into parts, export each, then import those exports (they're just clips) to create the final.
Say you made five exports. Import them, merge them on the timeline, export again. Might not save time in the long run, but it may lessen processing load and be less prone to export glitches.
Dealing at once with five-hours of content - especially if it's effect/transition heavy or the source files are HD - is a lot to ask. This is likely related to your "generating preview" post.
But that is only my opinion.