My odbc connection string for connecting to DB2i looks like this:
Driver={Client Access ODBC Driver (32-bit)};system=xx.xx.xx.xx;dbq=LIB1 LIB2 LIB3 LIB
This works as documented in the manual:
The library list is used for resolving unqualified stored procedure calls and finding libraries in catalog API calls. ... Note: The first library listed in this property will also be the default library, which is used to resolve unqualified names in SQL statements.