Is it possible to upload a list to use but have it not synch into Salesforce.com?
Yes, adding a lead to Marketo never automatically pushes it to Salesforce. Any lead creation in SFDC from Marketo will always be triggered wy a user action. Eg, all SFDC related flow steps will push the lead to SFDC.
But pay attention, some of these actions are well hidden: if you add a lead to a program that is sync'ed to a salesforce campaign, the lead will be pushed to SFDC