How can I import a bootstrap webpage on OutSystems. Let's say I did my frontend using HTML, CSS and JS somewhere and I would like to import it on Outsystem to use as my frontend, how can I do this? 

So, I have to ask...what are you trying to achieve here? Not re-creating the UI? Is this a single screen that you want to re-use, or part of an existing application?

You can re-use external content using IFRAME or similar, but importing an external screen is not a supported use case.

If you provide more details, we might be able to help you with an alternate path.

I am trying to build a notification icon with number of unread message just like Google+. I searched the Outsystems forge but couldn't find the best fit so I thought about building it with bootstrap and importing it.
I would be glad to know how I can achieve this. Thanks

For a clear picture, below is an example but it's deprecated:
https://www.outsystems.com/forge/component-overview/504/google-icon-notify-widget

Even if a Forge component is deprecated, you could still download it and review how they were doing the widget, as a means of replicating it yourself.

If all you are using or interested in is CSS styles, you should be able to create a web block and apply the desired styles to get the look and feel you want.

Just following up...I had a look at the Google+ Icon Notify Widget project, and the implementation appears pretty straightforward, though the CSS class names are a bit obscure. I'm guessing the author may have done some copy/pasting of the CSS and put together a container (div) hierarchy to replicate the look and feel. But again, you could review the implementation for ideas, and perhaps improve on it.