If you cannot (or don't want) use TIMEFORMAT your just need to transfer time into seconds, then add it together. For example pipe output through:

{                                           
sum=0
while IFS="[ :]" proc_name h m s
do
    let sum+=$((60*($m+60*$h)+$s)) 
done 
echo $sum  
} 

If you cannot (or don't want) use TIMEFORMAT your just need to transfer time into seconds, then add it together. For example pipe output through:

{                                           
sum=0
while IFS="[ :]" proc_name h m s
do
    let sum+=$((60*($m+60*$h)+$s)) 
done 
echo $sum  
} 

Or if you'd like can exchange last echo-command by

printf "%02d:%02d:%02d\n" $[sum/3600] $[sum/60] $[sum%60]