i've created a database system for a client of mine where they had the same system, online registrations for a sporting program, if your webdeveloper is able to export each registration into a .csv file then it can be automatically imported into the database via a script, the way i did this was not a simple process, it was done by apple script but you can do it semi manually if you like
online registrations are added to a .csv file on the FTP site
user downloads the csv file from the website and removes the one off the ftp server
you can set an automated import of the file via a button
after its done delete the file
further registrations will automatically generate a new csv file on the website
in terms of mass emailing, you can get your email application to automatically open and insert the email addresses from the found set (if they're in a field), that part is very straight forward, categorising is simple
e-newsletters, not too sure what you mean by that one.
filemaker makes a lot of things like this very easy to use, i've been working with it since the start of the year and i constantly find myself learning new things
hope this helps
If you can setup a vpn or a dns link to that site with the permissions to read the tables (mysql, oracle, mssql) then you can do it with filemaker ess and SQL External data sources.
This would be a much better way of doing it surely?