Appending data on multi select field via SugarBPM

Hello friends, 


Have anyone tried to append data on a multi-select field using sugarBPM? I have a "trade show" dropdown list for our Leads module. multiselect

Some leads we imported from a tradeshow, we will mark the dropdown list on a specific value on that dropdown. I encounter some problems when there are duplicated leads from another tradeshow. So I created the multi-select field using the same dropdown list to update leads appears more than once. However, when the data is imported, it only allows us to overwrite the multiselect field but not remain the old data. I know we can achieve that with Mass Update, however, it will be much more convenient to do it via a process definition. 

Thank you for taking your time to review/answer my question, any help is appreciated

Michelle