How to insert or modify customised table in employee portal?

You don't do that through code. You set up the custom table in the EP setup, and then on Sharepoint you set up the right web parts. The functionality of that webpart will then allow you to work with that table. For instance you can hook up a custom master table in EP and put a Card webpart on sharepoint, which will then allow you to create, modify and even delete(I think) records, without writing any custom code.

Now when you are talking about expanding generic EP functionality (there's only a limited number of webparts available out of the box), then you'd have to either develop that yourself, or purchase a 3rd party product. Me I would go for the external product. The company that developed the original EP sells their own, extended, product. Browse around this forum and you'll find lots of references to a number of other ones too.