Is there a way to add new members to structure in C?

I recently learnt structures in c and have known much about it. So if i have a structure like this:

struct test{
  int value1;
  int value2;
};

Can I add int value3 to the structure without having to edit the initial structure's declaration? [or re-declare the struct if possible]

I searched across many search engines and StackOverflow.com itself but I didn't find any questions like this nor answers nor any restrictions.

If there is any kind of way doing this in PURE C, it would be helpful to know