[Exim] Migrating users

We're currently trying to migrate users from one mail server to another.
As the migration process is going to require some human intervention for
each user, we are looking to transfer users one by one.

What I am looking for is a director that will forward users in a list to
a different machine. I'm currently working along the lines of:

migration:
driver = smartuser
local_parts = someuser
new_address = ${local_part}@???

The problem we have is that we support suffixed usernames, e.g.
someuser-foo@???. I'm trying and failing to make this
work using a second director of the form:

migrationsmart:
driver = smartuser
suffix = -*
local_parts = someuser
new_address = ${local_part}@???

but the suffixed username doesn't seem to match the local_parts
condition.

Ideally I'd like the list of users to be forwarded to come from a file,
to avoid editing the config file directly as we migrate users.

Any suggestions?

cheers,

Paul