The contacts you import or sync via Salesforce will look like completely new subscribers in the All Subscribers list as they'll have different subscriber keys. Unsubscribes etc are processed at the subscriber key level - not the email address level, so you will need to do some work to sync the status values.
This is something you can control. When configuring your email sends, you will be able to de-duplicate on email address so an email is received once, regardless of how many times the email address appears in the sendable data extension/list.
As an extra note, it is possible to run a subscriber key migration that actually makes use of the back-end subscriber ID (not to be confused with subscriber key) that will allow you to retain open/click history etc but this generally requires a services engagement.
If you simply want to ensure that you're honouring subscriber status (unsubscribes etc) at the email address level (rather than subscriber key, which is what SFMC will do), this is possible using SQL to return Subscriber Keys where the associated email address has at least one unsubscribed record in the All Subscribers list and one that is not unsubscribed. If you do this after importing all your contacts, you should be able to update the records in the All Subscribers list using the output. Example query:
from _subscribers s1
inner join _subscribers s2 on (s2.emailaddress = s1.emailaddress and
s2.status != 'unsubscribed')
where s1.status = 'unsubscribed'