You could possible encapsulate the EJB details inside a Java bean, and then from Fuse ESB invoke the java bean.
Then you could develop this java bean and unit test it outside the Fuse ESB. Then when its working, you can then deploy the bean and the needed WebLogic EJB client JARs.
There is also a Camel EJB component
However the trick is when using EJB is the client side need vendor specific JARs, as well setting up a InitialContext to the remote EJB server in a vendor specific way.
Often using a platform/language neutral protocol can be easier. For example HTTP/REST with XML/JSon, or for binary protocols Google protobuf or something else, even plain Java serialization.
I already have a Web Service (OSGi bundle) and from within want to invoke this EJB service hosted in a remote JVM. You mean to say, write another bean that invokes this EJB client jar and then that bean in turn is instantiated in the Web Service.