Hi Sergio,
 
i was wondering if it was possible to install pier admin on an
existing pier site..

It should work but I haven't tested it on an extensive range of pre-existing Pier kernels. 
Questions:
1) When you load pier-admin into your existing Pier installation does you Pier site continue to work correctly, that is the site without the pier-admin wrapper?
2) If the answer to 1) is yes then it should just be a matter of editing /config for pier-admin and defining the kernel, admin user and password.
3) If the answer to 1) is no, then one way to proceed would be to export your existing kernel. Install a fresh version of Pier, pier-admin etc and import your old kernel.

Hope this helps, and let us know how you get along

Nick