Yes there is no database creation script for that
You can try to copy db/tiki-1.9-mysql.sql db/tiki-1.9-db2.sql
(or choose the db/tiki-1.9-xxxx.sql that has the closest syntax to db2)
I think it is the only thing to do to have the database appears in the install page.
After I suppose, you will have to fix some syntax in this script. Sorry I never used DB2.
If you want to join the team to take care of DB2, you are must than welcome.
These scripts are created by a shell db/convertscripts/convertsql.sh from a generic file