[Exim] domainlist question - second attempt

OK - asking again still looking for the correct answer that actually
works and trying to be more clear. What I need to do is something like
this. The ordinary way to do domain lists is:

domainlist preprocess_domains = lsearch;/etc/exim/preprocess

where /etc/exim/preprocess contains:

domain1.com
domain2.com
domain3.com

----- BUT ------

I need a domain list where the file /etc/exim/preprocess looks like:

domain1.com: contains
domain2.com: other
domain3.com: data

The domain list needs to behave with the second list exactly the way it would
behave with the top example. So - the question is:

domainlist preprocess_domains = ???????