Yes it is possible to use objects that are on a hidden QlikView document tab in NPrinting. Make sure you hide the tab after you create the report templates in NPrinting or you will not be able to see the objects listed. Note that a hidden tab itself (as opposed to objects on a hidden tab) cannot be used in NPrinting.
One way you can hide QlikView tabs (Sheets) is by going to Settings > Document Properties then select the Sheets tab, select the sheet you want to hide, click the Properties button and select the Conditional radio button and enter a value of 0
HTH - Daniel.