How the shell inserts characters at the beginning of each line in a file:

awk '{print "xxx"$0}' fileName

How the shell inserts characters at the end of each line in a file:

awk '{print $0"xxx"}' fileName

How the shell inserts characters into the specified column of each line in a file:

awk '$O=$O" xxx"' fileName

To insert characters before and after each line in a file at once:

awk '{print "xxx"$0"yyy"}' fileName