All the while I can only find how to convert from FBX to XNB but not the other way around. Does anyone know how can I do that?
XNB is a binary container for all your content in programs that use the XNA content pipeline, this includes models, but also textures, fonts, audio, and sprites.
While it is possible to extract information from an XNB file (see for example this plugin for paint.net that extracts the textures in an XNB file: ) I do not know of any programs that extract model information. You will probably have to write your own program to do this.
On another note, why would you want to extract models from an XNB file, did you lose the source models? Or are you trying to extract and use a model that you didn't make yourself? Because that is legally gray/black territory.
Googling, I found these descriptions of the file formats:
FBX (Filmbox) is a proprietary file format (.fbx) developed by Kaydara and now owned by Autodesk. It is used to provide interoperability between digital content creation applications.
...compiled data files (.xnb) produced by the XNA Game Studio 4.0 Content Pipeline build process...
Judging from those short quotes, I'd guess it's a case of data loss in translation. Not all data in FBX ends up in XNB.
EDIT: Microsoft has posted a sample XNB parser (along with the format spec) which could be a starting point for the converter: http://xbox.create.msdn.com/en-US/sample/xnb_format