If you truly have a string (ie. in a variable), then you can simply do:
sprintf(f, "%s%d", string, k);
where 'string' is the variable and 'k' is the int value.
If the string is a constant value (eg. "N" as in your example) you can simplify it to:
sprintf(f, "N%d", k);
where 'k' is the int value.
... and really, both of the above instances of sprintf() will really be sprintf_s() if you write code for me.