Is it possible to change the "Create Account" screen when you use Liferay?

I want to be able to Add them to an Organization, send a POST request to a third-party API and a few other things, where could I go to learn about something like this?

I have worked with Vaadin to make portlets inside Liferay before, is there anyway to deploy a war file and have it take effect here? Or is that not possible?

Any advice, suggestions, links would be really helpful,

Thanks!

I found this which was helpful:

http://www.liferay.com/community/wiki/-/wiki/Main/Portal+Properties+6.0.5