Our use case is the following:

 When agents take a call and open a new support desk ticket, our customers have a client ID which is a 3-4 digit number.  We want to lookup the CLIENT ID number so it populates the account name, if we do lookup for account name we risk entering bad data as many accounts are similar.   Have looked everywhere and have not found anyway to add a custom lookup field

Getting to the point where I may end up having to concatenate the client ID and account name as a work around.