Import Lead by Salesreps without knowing PartyNumber (RegistryId) rather knowing custom identifier

Summary:

Lead Import Context - Business Salespersons are tracking Account details with their one custom attribute 'Account Number' so whenever they find any lead to create manually then they search account name based on this custom field 'Account Number' and create leads in Fusion. But now they are having daily 150 to 200 cold calls and capturing leads and want to use import method to create Lead in Fusion. Now they have excel with columns 'Account Number' and 'Lead Name'. But Fusion don't recognizes custom field to locate account as to import lead we need 'Party Number' attribute which is