MBR + Grub

When using MBR on your disk then you can configure grub chain-loading:

{
  boot.loader.grub.enable = true;
  boot.loader.grub.version = 2;
  boot.loader.grub.device = "/dev/sda";
  boot.loader.grub.extraEntries = ''
    menuentry "Windows 7" {
      chainloader (hd0,1)+1
    }
  '';
}

Source: https://www.reddit.com/r/NixOS/comments/31lx3i/windows_and_nixos_dual_boot/

EFI

After setting up a 256mb EFI Partition dualboot should work out of the box (at least for windows10)

Source: https://zimbatm.com/journal/2016/09/09/nixos-window-dual-boot/