If you want to create your own 3D content for use in the IMVU catalog, you need to use a 3rd party 3D creation program like Autodesk's 3D Studio MAX or Blender. No matter what 3D program you use, you will have to be able to export Cal3D assets. Those Cal3D assets are what you would add to an IMVU Editor to create a product.
In order to export those assets, you typically need to install 'exporters'. IMVU currently supplies exporters for all 3DS MAX versions listed below.
To get these exporters, please click the download link below.
To learn more about exporting in general, please go to the Intro to Exporting tutorial.