You can not get your file in a format that will allow a 3.3 user to create new ADT objects. The best you can do is to SAVEAS to the 2000 format, and have your 3.3 user turn on the display of proxy objects. The presence of "future" objects will disable the ability to create new ones in 3.3.
The other alternative would be to export to 2000. That will explode all of the ADT objects, which is not likely what you want.
Keep in mind that if the 3.3 users upgrade to 2007, you will have the same problem, as there were file format changes between 3.3 and 2004 as well as between 2006 and 2007.
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David Koch
Autodesk Discussion Group Facilitator