Add an object file(.obj) into a shared library file(.so)

Question

I am using an open source program and there is a shared library file(.so).

And I would like to add my object file(.obj) into the .so file, but I do not know how to do it.

Could anyone let me know how to add mycode.obj to lib.so in Ubuntu?

I have tried g++ -o newlib.so -shared lib.so mycode.obj. This makes a .so file but I do not think it contains lib.so correctly.

Thanks in advance.