NumtoStr returns single value (Last Value, Selected Value if inserting array as first argument). It is not returning array of strings.
In AFL there is numerical array only.
So to get expected result simply use AddColumn as you do in your second to last code line. It is array result already.
Also may additionally read here on why no array of strings.
Another thing... please read below post and please follow forum rule to include code tags if inserting code into post.