Hi Harjot,
There are a couple of different ways in which this can be addressed depending on whether you were looking for bi-directional sync or not, but for the sake this discussion lets assume you were looking for bi-directional sync. If the data from Siebel on Premise can be exported into a CSV format, then a cloud based app/script could be written (provided your company wil allow for such a script to sit on a cloud based server (need to disuss security and approved hosting services, etc.)), then imported into Marketo using Marketo APIs and vice-versa. Appropriate field mappings would also have to take place. The updates could be event triggered or time triggered. This is one of the challenges of On-Premise (behind the firewall) system and integrating with SaaS applications such as Marketo. We have extensive experience with this if you want to have a deeper discussion. Hope this helps.
Regards,
Bobby Sharma
bobby@growthfusion.com