Well, if you don't need compression I suggest doing the following.
1. Read about the ZIP file format(see links below ).
2. Once you get familiar with it, establish your workflow. Most likely your archive will have the structure(when no compression is done:
ZIP header
File 1
...
File n

This is just a draft, since I don't know the real format of a ZIP file, but it should be that easy.
So, basically, what you have to do is compute a header for your input files, add it to the out file and then start adding the input files one by one.

Regarding ZIP format:
http://math.ucr.edu/~mike/zipattacks.pdf
http://www.info-zip.org/pub/infozip/doc/
http://en.wikipedia.org/wiki/ZIP_(file_format)
But mostly this:
http://www.pkware.com/documents/casestudies/APPNOTE.TXT
http://www.pkware.com/index.php?opti...=64&Itemid=107


Regards