Suppy the 'unstuff' command with password to Expander?

I have this script which encrypts a password into .sitx file. What I am looking for now is to use a similar method to “unstuff” using the same password. I don’t want the User to have to enter the password. I want Applescript to supply the password programatically (no entry by the User). Is there a way to supply the “unstuff” command with a known password to Expander?

Any ideas?

-- User defined variables.
set password_ to "My Password" -- Replace 'My Password' with yours. Spaces allowed in password
set newArchive to "Macintosh HD:Desktop:new archive.sit" -- Spaces allowed in file/folder names.

-- -- Add as many files/folders to the following list as you like.
set filesNfoldersToStuff to {"Macintosh HD:Documents:3days"}

-- End user defined variables.

-- Geeks only...
-- Don't change anything below unless you wear a pocket protector.
set unixPaths to ""
set password_ to "'" & password_ & "'"
set nameOfArchive to quoted form of POSIX path of newArchive

repeat with path_ in filesNfoldersToStuff
	set unixPaths to unixPaths & quoted form of POSIX path of path_ & " "
end repeat

set stuffCommand to "/usr/local/bin/stuff -p " & password_ & " -n " & nameOfArchive & " " & unixPaths
do shell script stuffCommand


I don’t have Stuffit anymore, because since Tiger(?) OS X provides the common and cross-platform .zip format.
As far as I can remember there is an unstuff counterpart command line tool.
In type

man unstuff

or, if /usr/local/bin is not automatically recognized

/usr/local/bin/unstuff --help

to get the list of parameters. I guess there is also a -p switch for the password