How do you take the photos such that the scratches don't show up on the aluminium?
An extra shaft with a bearing would work, but it's a lot of hassle. You could have one rigid coupling and one flexible to save a few pennies...probably no need for the extra bearing, not sure.
Why not put the shaft extension on the other end and move the ballscrew towards the stepper motor?

The simple answer unfortunately is you should have bought a longer ballscrew!