or use substitute.
substitute "mystring1" "s/.$/_IK/";
// Result: mystring_IK //
or you could do more complex too (and actually preserve the numbering as is if you wsih):
substitute "mystring027" "s/([0-9]*)$/_IK_$1/";
// Result: mystring_IK_027 //
or just ditch all numbering:
substitute "mystring027" "s/[0-9]*$/_IK/";
// Result: mystring_IK //
In general i find regular expressions are the most useful tool in most day to day tasks. A must to know for anybody doing any kind of string stuff. Even for people who only use word or excel benefit from regular expressions big time.