[CentOS] sed

Not specific to CentOS but I know you guys would be really helpful
anyhow.  Basically, I have a file which has been editted in the past
very similarly to the hosts file only now I want to use it as a hosts
file and need to run some fancy sed to massage the data into shape.
Currently, the data in the file is in the form of <ip address> <tab>
<short hostname> <space> <short hostname alias>.  In some cases there
may not be any aliases so the end of line would be right after the short
hostname (no space at the end either).  In other cases there could be
many space separated short hostname aliases.  What I have been trying to
do without success is add our domain name to the first string after the
ip address and tab character.  As an example,
 
== Before ==
 
1.1.1.1    foo
10.10.10.10    bar bar2
100.100.100.100    foobar foobar2 foobar3
 
 
== After ==
 
1.1.1.1    foo.contoso.com
10.10.10.10    bar.contoso.com bar2
100.100.100.100    foobar.contoso.com foobar2 foobar3
 
Any advice on how to pull this off?  Thanks.
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