in iReport it easy to link an existing Input control (Folder Input controls in Report Unit) - right mouse - "Link an existing input control"
Is this as easy in Jaspersoft Studio possible - I couldn't find the context-menu in the Report Unit.
Right-click the report unit and choose "New".
From there it will be obvious. (But I understand why you wouldn't think of it at first.) It a different way to think of things, but I think the new flow is better once you're used to it.
EDIT: Based on baumgaro's comment, it's clearly not as obvious as I claimed. But you need to choose "Link" when your goal is to link to an existing input control (or link to another type of object).