There exist a script already which reads the model in blender made by marius schradek,but it doesn't export the edits back into the model.
http://www.datafilehost.com/d/1f0b6e1e
I have attached a sample model file and a hex Workshop book mark file.All you have to do is open the model using Hex Workshop,then open the bookmarks file(yobj_bookmark.hbk)
and i have commented and bookmarked the offsets you need to make the exporter.
Functions i'm looking for in the exporter
1)Export the bones,u-v maps and modified mesh/vertex.
2)Function which allows to use objects from other yobj models.
3)Blender Script
Below i have attached the blender import script by Marius Schradek which is compatible with blender 2.4.9b