Using Java JAR file in .NET

I could be wrong, but I'm pretty sure that's impossible. The java byte code is different to the code produced to run on the CLR.

Snarky answer: Get the source code, and port it.

EDIT: A little poking comes up with http://sourceforge.net/projects/ikvm/, a Java Virtual Machine implementation for .NET. Not quite what you asked for, but it's probably going to be the best you can do.