Hi Jang,
CallClient doesn't work from ExtendScript to ExtendScript, as there is the same FDK Client used for executing different scripts. CallClient works only from one FDK Client to another.
So if you want to use CallClient, you need your own small FDK Client, which controls the external script and your script.
AFAIK it's not possible to use eval for jsxbin files.
If the external jsxbin has a command created to execute that function, you can crawl for the command name and execute it with the fcode.
Hope this helps
Markus