If you have BMC Software Administrative Products installed at your site, you can view the DDL of the INDEX. By the way why do you need the DDL of index..? If you need to know the combination of keys involved i think you can query the catalog tables. I dont know about db2lookup.
Question to Marcus:
Is db2lookup available on all the platforms..? I wanted to know whether it works on Z/OS platform..?
Any suggestions would be greatfull. Thank you