Making changes to existing Liferay plugins / portlets could be achieved via Ext plugin http://www.liferay.com/community/wiki/-/wiki/Main/Ext+Plugin
However ext-plugin applies changes to liferay's code so it's better to do it with hooks. JSP hooks are the easy way around if you could achieve what you intend to.
http://www.liferay.com/community/wiki/-/wiki/Main/Portal+Hook+Plugins
If it's possible to use liferay 6.1 I'd suggest you build you implementation on it as it supports struts action hooks as well to make changes to existing java struts action files
http://www.liferay.com/web/mika.koivisto/blog/-/blogs/7132115
All my attempts to import the code into Eclipse (we use the Eclipse Liferay IDE with the Liferay SDK) failed so far....
What exactly are the problems you are facing? You can import the source as java project. A basic guide is available for it http://www.liferay.com/documentation/liferay-portal/6.0/development/-/ai/set-up