adding letters and character in a column using awk or sed?

With sed:

sed -E 's/^([^[:blank:]]+)/\1N-H/' file.txt

• ([^[:blank:]]+) matches the first field (^), and put in only captured group

• In the replacement, N-H is appended to the first field, \1N-H

With awk:

awk '{sub("$", "N-H", $1)}; 1' file.txt 

• sub("$", "N-H", $1) substitute the end of first field ($) with N-H, this necessarily means an append operation, in place

• 1 is just a placeholder for true so that the (modified) record is printed

Example:

% cat file.txt                               
A3 124.406526 8.281803
A3' 125.000684 8.389677
K4 123.072842 8.166373
K4' 120.092553 8.269080

% sed -E 's/^([^[:blank:]]+)/\1N-H/' file.txt
A3N-H 124.406526 8.281803
A3'N-H 125.000684 8.389677
K4N-H 123.072842 8.166373
K4'N-H 120.092553 8.269080

% awk '{sub("$", "N-H", $1)}; 1' file.txt    
A3N-H 124.406526 8.281803
A3'N-H 125.000684 8.389677
K4N-H 123.072842 8.166373
K4'N-H 120.092553 8.269080

Briefly, the one-liner:

awk '$1=$1"N-H"'