Bolincho:
I am not familiar with the conversion product you mention in your question but I can tell you that in my many years of experience in systems development I have found that tools to do conversions or emulation of the kind you mention do not work very smoothly and require a lot of intense work to actually make the thing function. Specially with a product as complex as DotNetNuke. It sounds like an interesting exercise if someone would have the time to do it but I really recommend you to not deal with it. I guarantee you that it is going to be a big headache. The product you mention is intended to be used when you have an organization with possibly a big investment in Java servers and ASP.Net applications already and maybe available ASP.Net programmers instead of Java programmers. But if you are at the point where you are starting from scratch with a content management system for Java, I would recommend you to get a product similar to DNN that is developed in the Java language so that you don't have to deal with a major code conversion. Plus, you may be able to get a CMS product for Java for free, the conversion product from Mainsoft probably will not be free.
Also, I do have experience in the installation and management of Java server products like the Oracle Application Server for Web Forms, the WebCT Training/Academic package, and the Serena/PVCS Software Development Version Manager (which I finished installing two weeks ago, their latest version) and I can tell you that the Java server architecture is very hard to manage. I don't know what these guys are thinking but to configure each one of these products you have to hunt text files and edit them by hand. In the case of PVCS, you cannot even add a new project via the Web interface, you have to do it with the Windows client, and then, you have to login to the server anyway to restart their service just to activate a new project site, give me a break, this is 2008. Oh, and PVCS uses a "custom" version of Tomcat, so you cannot apply the regular Tomcat patches, which you have to install by hand by the way.
And if I may ask a question, why do you want to run DNN on a Java server??? Remember that if you have a Windows server you don't need to buy anything to run DNN for a site of moderate traffic, you don't even have to compile DNN by the way, just get the "Install" package and it is ready to go. Also, is the Java server running on a Unix/Linux box? I ask because if it is running on a Windows server you could run DNN without conversion on the same server side-by-side with the Java applications.
Hope this helps a little.
Carlos