in C can we add variable in to structure at run time?

I don't think this is possible in any normal way, structures don't really exist once the code is compiled. You're probably better off using a dynamic array instead of a structure, or adding a dynamic array to the structure for adding values to it. If they won't all be the same type, you can use void pointers.

e.g.

struct stu 
{
    int a,b,c;
    void** extensions; /* initialize this and add values here as needed */
};

/* ... */
stu s;
s.extensions = new void*[5];
s.extensions[0] = (void*)(new int(4));

But I think if your design requires you to do this, you should probably rethink your design.

With dynamic memory allocation, you may do whatever you want.
It is possible, for instance, this way:

struct stu
{
  int a,b,c;
};

int main()
{
  struct stu * pstu;
  pstu = malloc(sizeof(*pstu) + sizeof(int));

  pstu->a = 10; // init a
  pstu->b = 20; // init b
  pstu->c = 40; // init c
  
  * (int*)((unsigned char *)pstu + sizeof(*pstu)) = 40; // init 'd'


  free(pstu); // never forget to release memory
  return 0;
}

As you may easily recognize, it is not the cleanest way to do things.
(Neverthless Win32 API often uses a similar approach).