AFAIK there is no ODBC driver for HSQLDB. The closes you could come is a ODBC-JDBC bridge, but using those is usually not very pleasant (or stable. or fast…).
I wouldn’'t recommend it, but you can access the script file directly - HSQLdb ‘‘databases’’ basically consist of a series of SQL statements in the .script file (and, if it exists, the .log file).
You could just edit that file directly (take care - if a log file exists, it is executed after the .script file is read, so make sure you take that one into account as well.). It’'s a bit finicky, but I have done this on occasion.
All in all, I think the best way would be to write a server plugin. It’‘s really not difficult due to the nice API (and example source) that’‘s available. I’‘ve written a ‘‘remote register’’ plugin (to create users from a webpage, rather than from a chat client or from the admin interface), that might be part of what you’'re looking for? What version of the server are you using?