Leads will not be automatically added to SalesForce. Marketo-only leads go to SalesForce only when you use the "Sync to SalesForce" flow action.
However, if anyone on your list is
already in SalesForce and their email address matches the email address of someone you import to your static list, Marketo will include that SalesForce lead in your Marketo list rather than create a new Marketo-only lead. This way you can avoid duplicate email addresses in your database.
I'd recommend reading Marketo's documentation before you import: http://community.marketo.com/MarketoArticle?id=kA050000000KzWVCA0