Contacts Who Haven't Opted In or Out

Hey all, we are still moving over to Marketo from a different platform-- so this is a set-up question.

In our previous system a subscription could be True, False or Blank.  Blank being not set.  Marketo seems to only allow True or False, so blanks by default imported as False.  Does anyone know a way around this?

I'm asking because we used to supress a list of people who were opted in, but didn't want a specific type of email.  Now, people who haven't updated their preferences, such as customers, cannot be emailed if we suppress that  list.

Any thoughts?