Establish a standard STRING tag (let's say My_String).
Then use COP NDN.Data.ReadData[14] My_String.Data[0] 14
Then MOV 14 My_String.Len
If the characters come out in the wrong order (swapped) then you may have to use SWPB on the original data INTs as SWPB doesn't work on strings. Or you can just do a manual swap of the string's SINT pairs using a dummy intermediate.
I'm not sure if I have your source string tag correct - the image was a little fuzzy.