Stitching together private svn repo and public git repo

Short Version: 

Repo B is a completely linear repo that has been tracking public repo A for
some time.  A number of manual merges were done to bring new A releases
into B but there are no connections between the two repos.  (How) can I
stitch B to A to reflect this relationship?  I want to retain B's history
and leave myself with an arrangement in which I can pull from A into B
periodically?  Many thanks for your suggestions, Greg.

--
Long Version:

I have been tracking my modifications of a public project released as tar
balls with my own local svn repository.  My svn repo was completely linear
and I always merged in changes manually each time a new tar ball was
released.  This started to get painful.

Now I want to do better.  I have set up a public git repository ("A") to
track the tar ball releases from the public project.  I have converted my
svn repo with git-svn to a git repo ("B").

My goal is to end up with a (new?) git repo tracking the public repo (A) so
that I can pull in any changes and merge easily. Fine, but I also want to
keep the history that I have imported from my svn repository (B).  So my
question is, can anyone suggest some pointers for how to stitch together A
and B?

Right now I have been trying to pull B into A to splice:

A $ git checkout v1.91
B $ git checkout v1.91-manualmerge
B $ git pull --no-commit -s ours ../A

This looks right when I run gitk, but if I repeat this for a second merge
point then the previous merge seems to disappear from history when I bring
up gitk.

I haven't found any docs/wiki info that has enlightened me yet.  Any
suggestions, wisdom, links etc very much appreciated - I do want to try to
get this right now, so I don't have to rejig everything in the future.  Best
wishes and many thanks,

Greg.

-- 
Gregory Jefferis, PhD                               and:
Research Fellow
Department of Zoology                               St John's College
Downing Street                                      Cambridge
Cambridge, CB2 3EJ                                  CB2 1TP