Bagaboo of Budapest make us these cordura, convertable backpacks, specifically for the Bullitt.

Massive messenger style backpack that converts and expands over the whole cargo area. The cargo area can be zippered out from the front part of the bag. The backpack has two large outer side pockets with flap and two zippered pocket, 1-1 on each side.
Inside there are 2 mesh pockets with velcro closure on the top.

The back is padded and has removable waist and chest straps – and also has a plastic insert which keeps the bag shape when empty. Side and bottom compression straps helps to keep the bag in shape even when there is no full load inside.
The top of the bag closes with a zipper and there are 2 mesh inner pockets for organizing.
The backpack is made out of durable cordura outer and waterproof inner tarpaulin layers. The expandable area is made out of TPU.

Dimensions:Bottom: Width 41cm x depth 25cm x height 60cm (closed) / height 85cm (open)Inches bottom: Width 16.1” x depth 9.8” x height 23.6” (closed) / height 33.5” (open)
Capacity / Backpack mode:
61 liters closed, 87 liters fully packed (3700 cu.in. closed, 5300 cu.in. fully packed)
Capacity / Bullitt cargo bag mode:
110 liters closed, 137 liters fully packed (6700 cu.in. closed, 8300 cu.in fully packed)