Steve,
3rd party libraries can be "OSGI-enabled" by using a tool like bundlor, see the FAQ "How to package a library as an OGSI bundle?" in the SDK docs.
Another way to solve that problem is to "inline" (or nest) the library inside your own plugin bundle:
1) Package your plugin-service.jar to include the library at the root level of the .jar file for instance.
2) Add the following line to your java bundle's manifest:
Bundle-ClassPath: ., my3rdPartyLibrary.jar
This tells the osgi framework to look for resources first in the current bundle (hence the "."), and then in the embedded my3rdPartyLibrary.jar
You can nest several libraries like this using a comma separator, and you can also keep them into sub-directories as long as Bundle-ClassPath uses the correct relative paths.
The advantage is that you don't need to modify 3rd party libraries that are not OSGI enabled, and deal with Import-Package.
The drawback is that there may be a memory and performance cost at runtime because the framework will have to expand the inlined jars, etc.