Hello, I couldn't find better way to do, but here is my solution, that worked for me.

1. I inherited purchase.report model

2. I've defined my custom field in it. Same field I have in purhcase.order.line as well.
   Example: dry_nuts = fields.Float('Expected Dry Nuts', readonly=True, group_operator="sum")

3. I inherited purchase pivot view and added my custom field.
   Example: <field name="dry_nuts" type="measure" />

4. In python file, where I inherited purchase.report, I copied complete query from purchase. report file and added additional query for my customs field.
   Example: sum(l.dry_nuts/u.factor*u2.factor) as dry_nuts
   

After that, dry_nuts are visible in purchase pivot table.

I think there is a better way to do it using ORM, so that there won't be need of copying all query from purchase.report, but I couldn't figure it out.

Hope this solution helps you too.