Merge two repositories

I have two tree-less shared repositories (format 2a) (lets call them A and B). A was the "main" project. Then I created B to research a particular topic. In B I have created some libraries about that topic. That libraries (and their source) are "generic", so I want to import B-trunk (and its history) in A-trunk. I'd like to import B-trunk as a subfolder in A-trunk (then I could move the files I need with bzr mv and delete with bzr rm). I'd like to maintain the history of B.

To make an example:

A->trunk->root(folder 1, folder 2, folder 3)
B->trunk->root(folder 4, folder 5, folder 6)

And I want:

A->trunk->root(folder1, folder 2, folder 3, folderForB->(folder4, folder5, folder 6))

Now something more difficult... At a certain point I have branched B-trunk, creating a B-branch (it's the "second option" for the B library... In researching the subject I found two equally good possibilities... Only the future knows which one will be the best)

So if it was possible I'd like to import the B-branch (with its connection to the B-trunk), perhaps as a branch of A-trunk

I'm not interested in maintaining the B repository afterward (it will be a component of A). I won't modify B separately from A. I don't want to keep the B repository separately.

Thanks