You can enter the text here (please see the screen capture): http://screencast.com/t/ZUXRtt48aq
and it will be applied to all selected products.
However please note, that adding any data in the Mass Product Changer will replace the existing descriptions.
In case you would like to merge the existing descriptions with some phrase you may use the SQL expression:
CONCAT ([FIELD(description1)VALUE], 'some phrase')
Please see the screen capture: http://screencast.com/t/ftzfXeTkKn
The Mass Combination Changer does not allow to change the combination weight, however you can generate the combination weight using the Product Combination Generator: http://screencast.com/t/Vca6Beb5
Also the combination weights may be imported while product import.