Is it possible to combine multiple frames into one still Cycles render to simulate long exposure?

Question

I have a simple 20 frame animation. The objects in the scene are imported png files as planes.

I have the various planes scaling and moving over time - Is there a render option that allows me to capture this movement in one output image?

20 frames long exposure rendered as one flat tif file.

I have the render sequence set at 1-20, Step = 1

Motion blur and deformation are active for the moving objects.

All I can get is one frame rendered and I would like to avoid post-composition of multiple frames by hand.