How to create OSGi bundle from jar library?
In principle you just need to add OSGi metadata to the manifest
There is a bundle creator for eclipse which gives a very practical way to add these entries which should be part of the Plugin Dev Toolkit.
Here is an article detailing the process and how to do it with the Bnd tool, maven and so forth.
I personally like the pax tools very much. It is all command line based, but very practical. To create an OSGi bundle of an existing jar you can use bnd tool.