I'm wondering if it's possible to import a list of leads (some new, some already in the system) from a webinar that would then create the lead in a SFDC campaign with a member status that I include in my CSV file.
Yes you can do this, but you need to import them into a Marketo program that is synced to a Salesforce campaign. You will also need to specify a "Channel" for the program that has the appropriate Marketo progression statuses (SFDC campaign member statuses).
You don't have to specify the member status in your CSV file as you can specify it for each batch you import.
If you don't want to import different batches based on the status, then you would need to 1) add a field in your lead records that would hold the progression status value, 2) create a smart campaign that would be triggered by that value being added to a field on the lead record and would change the progression status, and 3) add a column for that field in the CSV file and specify the desired status for each lead you import.