This functionality is provided by ioFTPD's user and group modules. Either use an existing module or write your own to share users between sites. Writing a script to catch SITE commands is just not practical, since you would also have to monitor uploads, deletes and other scripts which modify a user's credits and stats (nuke, unnuke, pre, etc.).
I'd suggest using ioShareDB with Harm's ioShareDB Helper script to fix a few of the bugs.
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