How to use and modify a public member data which was declared in another fix command class by using pointers?

Please keep the list in the loop. The fix ID is assigned
by you in the input script. It can be any string of characters you wish.
I suggest you grep the code for other uses of Modify::find_fix() to
see how it is used, or look at the code yourself. It's about 5 lines
of code in modify.cpp.