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VirtualBSD is somewhat compatible with VirtualBox, but not right away and not easily -- not to mention the fact that we don't have the resources(like this) to maintain and seed two different distributions!
However, we've prepared these instructions to help you with the conversion process:
1) Download VirtualBSD-90.zip and unpack it;
2) Start VirtualBox and create a new virtual machine as described in the following screenshots (taken on Mac OS X, so don't be put off by the cosmetic differences);