Yes! If you want to import a file of leads to an existing list, go to Manage -> Lists and then choose Import from the context menu next to the list you want to import into.
Context menu with "Import" item highlighted.
During import, you'll be given the option to update existing leads. If you do this, be careful to choose the right field used to match up leads. If an existing lead matches a new lead, any existing data will be replaced with data from the new lead. State including category, schedule, claims and activity log will not be changed.