An improved answer
On 64-bit systems NSInteger is long. On 32-bit systems NSInteger is int.
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Cocoa64BitGuide/64BitChangesCocoa/64BitChangesCocoa.html
All you need just cast your int to NSInteger.
int i = 1;
NSInteger nsi = (NSInteger) i;
You can also cast NSInteger to int (as in an original answer), but you must be careful on 64-bit system, because your NSInteger can exceed int limits.
An original answer
int i;
NSInteger nsi = 1;
i = nsi;
No big science. :)