Vikas,
I agree with Sam and John. If you need to update or insert Siebel data, direct database access is not the way to go. Writing your own adapter is possible. However, the effort to develop and to maintain it in the future to support new Siebel features and versions would make that option very unattractive from a cost standpoint. You might not have to pay webMethods for a Siebel adapter license, but you would pay many times that price to write, debug and maintain your own over time, IMHO.
In some (maybe all) customer contracts, writing directly to the database layer gave Siebel grounds for dropping support for your account.
Another option that has not yet been mentioned is to use the web services layer provided as part of more recent Siebel versions. This is more work than using the adapter as I believe most of the provided services are the equivalent of low-level API calls, but it would work and it would be supported by Siebel.
Mark
#Integration-Server-and-ESB#webMethods#Adapters-and-E-Standards