Contacts can be imported from a CSV and exported again from the header of Contacts.
After Importing
Imported contacts have no behavioural history until the snippet sees them on your site. They will appear in Contacts immediately, but with empty page-view and last-seen columns, and they will not match behavioural audiences until they visit.
Your contact count is a plan limit checked against the live total. A large import can push a workspace over its limit — check Settings → Usage against the number of rows in your file first.