Hi Prasanna,
There is no hard and fast requirement for getting certified in PL/SQL if you want to learn BI and HANA. Certification is good when it is demanded or to highlight your CV. Since you already have knowledge and experience with PL/SQL you can then learn HANA and that too free of cost from (www.opensap.com), go ahead and create a profile in opensap and you can take the training and exam. This is a very good opportunity so don't miss it.
For career in SAP, keeping in mind your interest to work with DATA and HANA you may learn BI that will let you work with data from different data repositories and also work on HANA as it is the default in-memory DB used with BI now a days, it will also get you good exposure and you may get good job opportunities also.
Cheers,
Nagarajan Viswanathan