control the UID/GID assignment by installer

[Ross Boylan <rossboylan@stanfordalumni.org>]

I would like to control the UID/GID assignment to match that of
another system.  E.g., UID 101 was Debian-exim, but on a new install
that's the UID of messagebus, and Debian-exim is 103.

I think I can do a minimal install and then modify /etc/passwd for any
accounts not yet created, assuming they don't overlap with already
existing ones.  But is there a way to intervene before then?

Thanks.
Ross Boylan