is there anyway of addind a custom column to the Reallocate Items page?

Not natively. Pages that can be customized will display the Customize button.

Yes, so there is possibly a way to do it un-natively? We use the page a lot due to our internal processes and really need to be add things like the Customer Ref/PO No. as a column.

You would need to speak to a developer. With that said, I am not sure what Customer Ref refers to, but Customer already displays on that page. Even a developer would struggle to show you PO, since allocation refers to Sales Orders. If you are using this regularly, you may want to ask for a demo of Supply Allocation which affords some advanced allocation rules.