Following wiki page is a little bit outdated but the main points are the same. Anyhow, this isn't technical issue but if you directly change liferay's source code, you'll have to make the source code available with the binary distribution according to liferay's license.
http://www.liferay.com/community/wiki/-/wiki/Main/Building+Liferay+distribution+files
BTW, there's already a team that's providing the patched version of 6.1.1 GA2
There's also this post too. It's probably would be easier if we can all work together to improve quality of liferay rather than trying to do everything by oneself.
http://www.liferay.com/community/forums/-/message_boards/message/16790273?_19_delta=20&_19_keywords=&_19_advancedSearch=false&_19_andOperator=true&_19_resetCur=false&cur=1
Por favor, identifíquese para poder marcar esto como inapropiado