Hello Rudi,
Thank you very much for your help and your tips!
Now, I could solve my problem in two different ways.
1. There exists already an overlay for Barnard dark nebulae, which can be downloaded here:
http://www.projectpluto.com/extras.htm#bdn2. I created a new tdf file based on Bill's LDN.tdf
My new tdf file is named _LDN_Barndard.tdf and contains only those LDN objects which have one or more Barnard numbers.
One can download the zip file and unpack the two files (_LDN_Barnard and _LDN_Barnard.tdf) in the Guide root directory.
https://dl.dropboxusercontent.com/u/46345993/_LDN_Barnard.zipbest,
Bernhard