It is very easy to do. As long as you have the string built that you want to execute using the REF CUSOR before you OPEN it, add whatever results from other procedures you want. So you would have something like this:
my_var := my_Function();
my_sql_var := 'SELECT something, ' || my_var || ' FROM my_table';
OPEN my_cur FOR my_sql_var;
FETCH my_cur INTO ...