David,

If the first 2 lines of the "input file" contain the password and then the
list of files names it works.

For example if my password it "abc" and I want to add "t.txt" to "t.zip" I
would make my input file equal to

<snip>
abc
abc
t.txt
</snip>

and then execute zip like

/tmp>zip -@e t.zip < i
Enter password:
Verify password:
  adding: t.txt (deflated 84%)
/tmp>

Good luck and don't hesitate to contact me if you need any further
assistance.

Regards
Jens