Bring the PDF into Sierra and select the Job Template used for this layout.
Add the Pages to the Reading Order, and then Select all Pages that will need to have Creep applied.
In the Job Page Properties, Check "Override Creep" and then select "Creep Adjustment for the Face only." 
Select the Plate workflow item. Select and edit a template or create a new template. In the first tab (Imposition) set the Default Creep to the paper thickness. - It must be a negative value. 
On the third tab (Sheets) select “Manual (Use Default Creep)” from the Creep drop-down menu and click on update.
Pages will now scale toward the spine, instead of shifting the pages into the spine.
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