CIFS Prepop is a SteelHead function to warm up the Datastore (SteelHead cache).
If you want to use SteelFusion and warm up the Blockstore (SteelFusion cache) you can first PIN the LUN and let users read and write data against the LUN. After a while you can UNPIN the LUN and you'll have warm data in the local Blockstore.
If you just PIN the LUN without Prepopulating it will not start to sync all data from datacenter onto Edge appliance.
You can't PIN a LUN that is bigger than 90% of usable Blockstore cache.