Is there a feasibility to show an external html file within an siebel applet? If yes, I need to know which class I have to take, which type of field, etc.

Displaying external Web content in a Siebel applet can be achieved through Symbolic URL's. This technique allows to display a Web page either as inline HTML, within an IFrame, or within an embedded Web browser control.

This is documented in Siebel Bookshelf 8.0 > Siebel Portal Framework Guide > Integrating External Content.

There is also a document available on My Oracle Support which discusses how to embed an external Web page through a Web control:

Applet containing an external web page in Siebel 7 (Doc ID 543107.1). It was written for Siebel 7 but should still be applicable to Siebel 8 and might serve as a starting point.