There is no special requirement / configuration required for DB2 and is the same like other Databases. You just need to install the DB2 client on your Campaign marketing server - from where you want to connect to DB2. For downloading this driver please refer to:
After you install the Access external data package on your campaign instance (Tools>advanced>import package) , You need to create an external account of type : External Database and Connection type: DB2.
Then fill in the required information as shown in the screenshot attached and should be able to connect.