Download the asset in 3.6. For an asset with a build number higher than the build number of the eventual asset, you'll have to open the asset for editing, and edit the build number in the asset to be equal to or lower than the build number of the target installation. When you've edited any needed assets, save the desired assets to one or more ~.cdp file, and save them in a known location (like the desktop). Close CM 3.6, and open the version of CM associated with the earlier version of the game. Open the content manager, use the option "import ~.cdp's". Note that while this will work with most assets, if you're trying to backdate an asset which depends upon a feature of the game which was introduced after the build number of the target installation, the asset may not work, or may not work as intended.
ns