Programming :: How To Add Characters And Lines To File

For example, I have a file called "file" like this one:
type=strongsubj len=1 word=absolve pos=verb stemmed=y priorpolarity=positive
type=strongsubj len=1 word=unique pos=adj stemmed=n priorpolarity=neutral
type=strongsubj len=1 word=absolutely pos=adj stemmed=n priorpolarity=neutral
type=weaksubj len=1 word=taking pos=verb stemmed=y priorpolarity=positive
type=weaksubj len=1 word=friend pos=noun stemmed=n priorpolarity=positive
type=weaksubj len=1 word=usually pos=adverb stemmed=n priorpolarity=positive
type=strongsubj len=1 word=purecolor pos=anypos stemmed=n priorpolarity=negative
type=strongsubj len=1 word=accusingly pos=anypos stemmed=n priorpolarity=negative

I want to add the plural for the noun, for example if find this line:
type=weaksubj len=1 word=friend pos=noun stemmed=n priorpolarity=positive
will add one more line :
type=weaksubj len=1 word=friends pos=noun stemmed=n priorpolarity=positive
where we add "s" for the word friend
I did try to do like that:
<code>
cat file | while read LINE ; do

set -- ${line}
if [[ "${4#pos1=}" == "noun" ]];then
#I tried this line but it doesn't work properly:
v3==$(echo $line |sed 's/$3/$s') #I want to find the third word "word=friend" in that line and add "s" after that word
# I don't know what command to add this new line "$v3" to the file ???
done
</code>