http://naruto.wikia.com/wiki/Third_Eye

Make a personal variation of that jutsu.

Doujutsu do not inherently have to be Kekkei Genkai, no. As a matter of fact, before Sai ever starting stealing Kekkei Genkai, his father had a jutsu called Sage Transformation: Hawkeye. It was a doujutsu similar to, but weaker than Byakugan and would work as long as he held the rat symbol in his hand. The issue with doujutsu is that they literally morph the body, and doing that normally requires some form of Kekkei Genkai or advanced medical knowledge. In the case of Sai and his father, Talon, they had the Jugo Clan Kekkei Genkai, and they used Sage Transformation to do it.