SQL & PL/SQL :: Populating Ref Cursor From Collection?

I have an existing Procedure that uses a ref cursor as a parameter. The procedure currently simply selects data into the ref cursor using an OPEN FOR and then exits.

I have a need to extend the processing of the procedure. This will mean retrieving extra columns in the select and also removing some of the retrieved records.

I have the constraint that the definition of the REF CURSOR (which is based on a record type as follows) cannot be modified.

TYPE t_charge
IS
RECORD (
id number,
date_from invoice_detail.from_date%TYPE,
date_to invoice_detail.to_date%TYPE,
description invoice_detail.desc_text%TYPE,
amount invoice_detail.amount%TYPE
);
TYPE t_charge_cursor IS REF CURSOR RETURN t_charge;

The procedure does this

PROCEDURE get_bill_lines (
p_bill_id IN NUMBER,
bill_lines_list OUT t_charge_cursor )
IS
OPEN bill_lines_list FOR
SELECT id, from_date, to_date, desc_text, amount
from invoice_table
where id = p_bill_id;

I am thinking instead of selecting directly into the bill_lines_list parameter I will use a local REF CURSOR to select into a COLLECTION. I can then do some work on the collection, similar to below:

PROCEDURE get_bill_lines (
p_bill_id IN NUMBER,
bill_lines_list OUT t_charge_cursor )
TYPE t_local_charge
IS
RECORD (

[code].....

This all works fine, but the question is, how do I then put the collection into the bill_lines_list REF CURSOR where the '*** HERE ***' comment is so I can send the existing REF CURSOR structure back? (Remember I cannot change any definition that is referenced externally)