Hi,

You can  override the _prepare_invoice_line_from_po_line method in account.invoice model and pass your custom field values from to account.invoice.line.

If you need to pass the value from the purchase.order to the account.invoice, either you can pass it in the context of the Invoice smart button in the purchase order form or you can fetch the value from account.invoice model using the purchase_id field in the account.invoice model .

Thanks