I would be quoting http://dunkels.com/adam/dunkels06runtime.pdf in this question which presented the idea of dynamic linking / loading by Contiki.
The loadable module can have references to the functions or variables in the core. These references are resolved through the Contiki symbol table (containing symbolic names of funcs/var in the core and their respective addresses).
My question is whether it is possible to have a loadable module make references to the functions or variables present in another loadable module? And if that is possible, then how are these references resolved at run-time while Contiki Symbol Table, apparently, contains only its own symbols.
No, it is currently not possible. You can of course add a symbol table to each module. The liker will than fill that table during linkage. You than only need to extend the linker to not only search the main symbol table, but also those of the installed modules.