Yes you can. You can import them into a static list on the Lead Database tab, or you can import directly into a Program. Once in Marketo, you have the option to sync with SFDC or not. Right click on any static list and select import. Then, follow the import wizard. In the third step, make sure you reference the right smart static list. Then, under acquisition program, reference the program you are importing into (if choosing that option), or if it is just a general list, choose "None" as the program. All depends on where the list came from and what you are trying to accomplish.