As far as I know, that is not even possible in Siebel. At least, not easily.
The only thing you can add to a Siebel view are applets. Inside an applet, you can embed "advanced" controls like an ActiveX object, a Java applet, an external web page... but they wouldn't have the appearance, nor the behaviour, of a floating window.
Via server scripting, you can open popups, raise error texts, and that's all. In browser scripts you have a few more options (alert, confirm, etc), but nothing useful for your requeriment.
Maybe you could tweak the webtemplate files and insert your floating window HTML code there... and use browser scripts to show/hide/control the window. But then, you could have only HTML, you wouldn't be able to add fields to it. Also, it would be a total pain to make it work, and a maintenance nightmare since the very first moment.
If you were using Siebel 8.1+, you could try the new OpenUI, instead of the standard ActiveX interface. It's based on CSS and jQuery so it should be easier to tweak the appearance. But it's only available for Siebel 8.1+.
Do you really need a floating window? Why don't just use a parent-child view with two applets?