How to add an extension to all files via terminal

for f in * ; do 
  mv "$f" "$f.zip"
done

rename 's/$/\.zip/' *

Don't use xargs for that!

Searching - few links:

1. Recursively add file extension to all files - Stack Overflow
2. Add file extension to files with bash - Stack Overflow

man rename:

NAME
       rename - renames multiple files

SYNOPSIS
       rename [ -v ] [ -n ] [ -f ] perlexpr [ files ]

DESCRIPTION
       "rename" renames the filenames supplied according to the rule specified as 
       the first argument.  The perlexpr argument is a Perl expression which is 
       expected to modify the $_ string in Perl for at least some of the filenames 
       specified. If a given filename is not modified by the expression, it will not 
       be renamed.  If no filenames are given on the command line, filenames will be 
       read via standard input...

man wiki: http://en.wikipedia.org/wiki/Man_page

A very simple way to do that is :

if you want to keep current extension:

for i in *; do mv $i ${i}.zip; done     

if you want to replace current extension:

for i in *; do mv $i ${i%.*}.zip; done