Import existing (old) topic branches in topgit

Hi all,

After trying to manually maintain many topic branches before sending them upstream, I recently discovered that topgit seems to become my best friend.

But as I'm new to this tool, I don't know what's the best way to import existing branches in topgit.

What I mean is:
* I already have topic branches, with long history and regular merges
with upstream.
* I want to manage the related topic via topgit
* I whish to keep my own history (if possible)

Can experimented users give me some tips on how to do this?

Thanks in advance.