Since Marketo uses email address as a unique identifier, there is no way for you to later update those records. Unless someone in your CRM updates them to add email addresses. Without an email address, they are lost to Marketo, although Marekto can create the records and sync them to CRM.
So lets say your list today has Joe Smith, CTO, Acme Corp. If tomorrow you upload a list or Joe Smith fills out a form, Joe Smith, CTO, Acme, joe@amce.com will become a duplicate new record.