extend items in invoice template through hooks / events

I want to add some more attributes to each basket item before creating the invoice PDF through an existing plugin. The plugin uses a hook like onbeforesendmail. Can I use this to extend the basket items which are then displayed in the pdf?

Which sOrders hook / event should I use?

How can I extend the orderitems variable for use in the template with more attributes through the hook?

Through setreturn?

Have to update the $position in the sent mail PDF