Diego, thank you for the ping (again :-).
Xavier, it is a pity that modifying a datasource is not an option.
The solution is as simple as making a range join by date (between ... and ...).
Another approach is using a data blend (with a hand-made "Scaffold" DS).
It is rather complicated (as compare to a range join).
Please find the attached wb (version 9.0)
and CSV files used to make a Scaffold DS.
Hope it could help.
We can define the CUSTOM RANGE
if Datediff([ 'day' , [Start Date], [End Date]) <=30 then " 1M"
elseif Datediff([ 'day' , [Start Date], [End Date]) > 30 and Datediff([ 'day' , [Start Date], [End Date]) <= 60 then " 2M"