Since I have Java, C# and C++ versions of my open source program (
http://www.euclideanspace.com/ ) I would like to have some automation
to help with the manual conversion between these types of source code.
I would also like some automation for generating UML diagrams.

I had this idea that this could be done by storing programs in a
common format, call it say: "XML structured Java", once its in this
form we could then use XSLT and similar tools to convert between the
different source codes. I had a search on Sourceforge to see it I
could find any existing code and it looks like someone else has had
the same idea: (http://sourceforge.net/projects/xmlcode) but nothing
seems to have been done on this project for the past 4 years. Do you
think a very quick and dirty version of this could be implemented
quickly?

I had another search on Sourceforge for programs which might provide
the functionality required and I came up with the following diagram:
http://www.euclideanspace.com/software/language/convert/index.htm

Is this practical?

Are there any better ways to do this? Should I use Eclipse instead?
Are there any better GPL open source programs?

How much 'glue' programming would be required to make all this work
together, not perfectly, but enough to make it useful for source code
conversion.