The problem is that you want to store the imported extra attributes directly into your .max .mb or .blend file for later data exchange. So if adding properties is not supported in some cases then we (the developers) have to find other ways of storing the data.
Some of my ideas:
Blender allows script links for material and scene objects. This would allow artist to link script files to a node object or to a material. The script file could be in any format and the text file could contain any information the user wants. In any case this text should provide enough information so that when I export I can put extra elements and data where I have decided to put them into the collada document.
My idea is that the artist create a dummy scene node object (in blender called Empty) and put it in my node hierachy. I can name the node for example "COLLADAExtraData". I then add child nodes that have names that corresponds to the names of the objects (data or instances) that I want to add properties to. These child nodes have scripts links which contains xml text information that should be added as an <extra> element to the very same object that was stated as an ID-reference in the node name.
When exporting the data I can get the "COLLADAExtraData" node and with that information find its children. Whenever I'm exporting a xml element in my exporter I can see if I encounter an ID match with my dummy nodes and the ID-reference the xml element will have. If I have a match then I add the information in the script link into a <extra> element into the document.
The "COLLADAExtraData" node will not be exported into the final document since the information here has already been preserved into the document.
The COLLADA importer then simply creates this node system whenever I encounter an <extra> element.
That was a bit implementation specific for Blender but other DCC tools can perhaps use the same node system for storing extra data. Even if a link system is missing for nodes there should be some way of storing text information in objects other then nodes.
To allow artist to add and/or modify extra properties an GUI can be scripted.
On a similiar topic most modelling software have features that can be exported into collada but their different content creation philosophies makes it hard to get a perfect data exchange between tools. Maya cannot understand light falloff data and Blender cannot understand polygons with more than 4 vertices. Storing such data between DCC are not hard. It's impossible.