I see that you never received an answer. I also have a need to have SLDDRT files managed in Windchill. However, according to the SolidWorks WGM Guide (http://www.ptc.com/WCMS/files/157343/en/WWGMSolidWorksGuide_10-2-M010.pdf page 62 ) this file type is not supported.
I have created a Product Idea (http://communities.ptc.com/ideas/4240) that is open for voting. Maybe if we can get enough people to indicate that they need or want to manage SLDDRT files in the workspace PTC will accomodate this in a later release.
10.2 version of sw wwgm incorporates the windchill file system - you can now store your slddrt files in windchill and then solidworks access them via a path location that "exists" in windchill - this might accomplish what you are seeking?
It might. That would simply have us load the file into Windchill like we would any other WTDocument, instead of like an EPMDocument. That might be an acceptable workaround, given that we should be able to path to the actual file location in Windchill. Definately a bummer until migrating to 10.2. If it works I will update this topic next month.