Haven't tried it, but what about the StyleGallery ?
e.g.
...
var mxDocument = application.Document as IMxDocument;
var styleGallery = mxDocument.StyleGallery;
styleGallery.LoadStyle(filename, null);
or,
var styleGalleryStorage = mxDocument.StyleGallery as IStyleGalleryStorage;
styleGalleryStorage.AddFile(filename);
See:
ArcObjects 10 .NET SDK Help - IStyleGallery
ArcObjects 10 .NET SDK Help - IStyleGalleryStorage