How do I associate a third party sale person to multiple accounts?

How do I associate a third party sale person to multiple accounts?

We have a large distributor sales team that works for a third party company.  If I enter each sales person in the CRM and assign them to their company as a contact, is there a way to also connect them to their customers they service for us.
 
I do not want to enter them as a users as they will not be using our CRM but have set them up as contacts within our system.  Is there a way to either assign them to multiple accounts?  Or is there a way that I can create a custom field within an customer account that I can fill with the sales person's contact information.
 
I am trying to figure out an easy way to associate an outside sale person with multiple accounts.
 
Your help is greatly appriciated.
Thank you,
Patti