Is there a way to transform a partylist field like Customer in a Campaign Response to allow for multiple Contacts/Accounts/Leads?

Although unsupported, I now from Jian Wang that some attributes of the lookup can be modified in the onload() event like this:

crmForm.all.customer.setAttribute("lookuptypes", "1,2");

Is there a similiar attribute that would turn this into a field that allows multiple participants like the To field in an e-mail?

Thank you