If you have got a web service that will access DB2 then you can call this using script from a Nintex form.
The SQL Request control can't access a DB2 database as it currently only supports SQL Server.
Potentially, if you can configure SharePoint to have an external data list, which would surface database data. If it's possible to set this up in SharePoint, then you could have a list that will show the data from your DB2 database, with read/write permissions. With that, you could then build a Nintex Form on that list.
Retrieving data ...