[exim] multiple dummy domains

We have a number of domains that are registered for multiple web sites. The
web sites are managed by a third party but I need to host the emails.

However, but don't want to have to go to the trouble of configuring EXIM to
work for all of these domains, and then setting up multiple forwarding
aliases.

Is there an easy way to change these domains to work on one of my existing
domains? Each one of these domains will have the same four localparts

sales@
orders@
quotes@
deliveries@

I have already set up these addresses for @existing_domain.com

Is there a quick way to make all

<whatever>@websitedomain.com go to <whatever>@existingdomain.com

I've got 4x localpart for upwards of 4 domains.