Forking off a remote public cvs. Howto merge the parallell development?

[Erik Sjölund]

We want to fork off an open source project where the
code resides on a public cvs. The forked code will then
reside on our own private cvs-server. We will work on
the forked code on our private cvs-server while the
development continues on the open source projects 
public cvs-server.

What possibilities are there to merge in new releases 
from the public cvs into our forked code in the future?

Are we forced to do it manually or is it possible to
merge with the help of cvs? 

Is there a way to do it by importing multiple times, maybe?

thanks & regards
Erik Sjölund