Of course you can create a separate connection to other database system in EPESI.
What I recommends is to install EPESI on mysql or postgresql and write a wrapper to your database. We use ADOdb library which should support your database (link here - search for "ibase" - it's a proper driver).
We've got some wrapper around ADOdb, such that if you would like to obtain something from EPESI database you can use DB class with it's static methods. To create a connection to your firebird database you'll have to create your own connection object with NewADOConnection and use it to query your data.
Please refer ADO documentation for some howtos, etc.
Regards,
Adam
