Adding data to a secondary field if target field already contains data

I have a specific use case where I need to have a contact mapped to multiple accounts (which I know isn't possible currently with Eloqua).  I work with a company that provides marketing services for over 500 independent companies, and those companies can at times can share customers.  We want to be able to send out company-specific emails to these customers, so we've linked our contacts to account information inside Eloqua - this way when a customer receives an email from us marketing services from a specific company, that company's name and logo populates the email that is sent. 

We've recently run into an issue where we have an ongoing campaign that has contact segments added once a week (appx. 15 contact segments are added each week).  We are pulling updated customer contact data from a report the week prior, and these updated lists are added into Eloqua contacts and the contacts are assigned to a specific company (via a company number) that maps to a company Account record.  When these lists are being uploaded (usually between 4k-5k contacts), there are some contacts that already exists in the Eloqua contacts database (because they were imported as part of an