The criteria to include a network folder in a Windows 7 library:
- A network folder must be added to the index or made available offline before it can be included in a library.
References:
Include folders in a library
Libraries: frequently asked questions
It is not possible since Windows 7 won't be able to index a network folder of non-Windows origin (incompatible file-system; Buffalo NAS is using Linux-based file-system that doesn't index in similar fashion as Windows-based file-system). Buffalo NAS is using XFS or EXT3 instead of FAT or NTFS.
If your concern is more about indexing files like music/movie files, you can use DLNA service available on some Buffalo NAS models.
Media Streaming with Windows 7