Q:

Can I set for example array[n+k] = array[0]?

If I have this code:
#include <iostream>

using namespace std;

int main()
{
    int n, i;
    cin >> n;
    float array10[n];
    cin >> array10[i];
}

How can I create array10[n+1] and set the value of array10[0]?

A:

You can't. In fact, array10[n]; isn't legal C++ (even if n was initialized). Variable length arrays aren't a C++ feature.
Use std::vector:
cin >> n;
std::vector<int> vec(n);
cin >> vec[i];

