karaf@root> packages:exports | grep useradmin
0 # org.osgi.service.useradmin; version=1.1.0
20 org.osgi.jmx.service.useradmin; version=1.1.0
82 org.osgi.service.useradmin; version=1.1.0 --> OSGi Compendium osgi.cmpn (5.0.0.201305092017)
89 org.apache.felix.useradmin; version=1.0.0 --> Apache Felix User Admin Service (1.0.3)
That first bundle you listed, 0, that is exporting useradmin, I suspect isn't actually exporting anything. The second one is exporting a completely unrelated package. The third one is exporting the actual useradmin API. And the fourth one is exporting the apache felix specific classes.
Karaf doesn't actually contain the useradmin package anywhere in the standard download.
apache-karaf-2.3.3 sartrip -> gfind -iname \*jar | parallel unzip -l {} | grep userad
0 01-23-13 14:59 org/osgi/jmx/service/useradmin/
4462 01-23-13 14:59 org/osgi/jmx/service/useradmin/UserAdminMBean.class
822 01-23-13 14:59 org/osgi/jmx/service/useradmin/packageinfo
0 02-08-13 11:24 org/apache/aries/jmx/useradmin/
12187 02-08-13 11:24 org/apache/aries/jmx/useradmin/UserAdmin.class
1828 02-08-13 11:24 org/apache/aries/jmx/useradmin/UserAdminMBeanHandler.class
That means you must install the a bundle containing the useradmin API, either by installing the OSGI compendium API bundle or building your own JAR containing just the parts you want (org.osgi.service.useradmin).
EDIT:
I'll also point out that the not-yet-released version of apache felix useradmin will contain the org.osgi.service.useradmin (as it should!) meaning eventually your dependency on the compendium API jar will go away. https://github.com/apache/felix/blob/trunk/useradmin/useradmin/pom.xml#L81