While true you cannot add the Java portlet directly to Sharepoint, Sharepoint, at least in the list of functions, is "W" eb "S" ervice "R" emote "P" ortlet client, or WSRP client. So, what what would you do is build your Java portlet and then deploy and run it in a portlet container that supports a WSRP server like Liferay ... I think this is the one that will do this. Once this is done, you can import the remote portlet into Sharepoint. For Sharepoint 2007, here is a link that may help.
MSDN Code Page for WSRP Tutorials for Sharepoint 2007
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