custom order filter

Is it possible to enter a custom PHP code that changes the order of products in the export output? Unfortunatelly the field I am trying to sort the order from isn’t available in the dropdown of the order settings.

The field is:
_pickup_hidden_datetime (format e.g.: 21-01-29|00:00)

I already have a custom PHP code to filter only ‘today’ on that same field. That is working great! But it would be nicest to also sort the bookings by time; so from the earliest bookings of that day to the latest bookings.