Combine forums? (Like, dump all into one and keep original + 'updated')

[nothing4me]

Let's say I have these forums:
1,2,3 (subforums) under forum 4 (1,2,3 are the children of parent 4.)

And I have another forum, 5, which is a child of another parent forum 6.
How would I combine forums 1,2,3 into forum 5 yet keeping forum 4.

• Forum 4
  • 1,2,3
• Forum 6
  • 5 (1 + 2 + 3 combined)

I want to keep 1,2,3 and have them all in 5 at the same time. So, 1,2,3 makes it organized, but if anyone wants to see it all together, they can go to forum 5.