OK!, so its a matter of automation.
If the descriptions are in the flat file from external system, then I would suggest to upload this file in BPC automatically! Follow this thread - Automate Flat file upload into BPC File system | SCN
For loading the ID's from BW continue to use the data manger package to load them, and then use the flat file to import descriptions! Use data manager package links to automate this process....
Hope this helps...
Regards,
JP