Maintain 'alien' project

I frequently enough bump into the following situation. I use a tool (a
project) which is designed by someone else. I make some corrections to
it, however the maker(s) does its own. It's not a GNU Arch project and
I don't have any access to the repository (usually it is a CVS). When
a new version is released it knows nothing of my changes and I have to
apply them manually. Can GNU Arch help me with that?

I know enough to be able to make import, branches, etc. (the basic
Arch functionality). What I'm not sure about is how to use them in
this particular situation. My intention is to organise my own GNU Arch
project which would allow me to accumulate author's changes easily
enough, at the same time keep my changes also.

If it were GNU Arch project, I might have made my own branch from it
and do star-merge as soon as new original version is
released. Unfortunately it is not.

Would you mind advising my something?

-- 
Vladimir Zolotykh