I assume, correct me if I'm wrong, that you mean his birth date is unknown to your family? I am a bit confused why you say you don't exist, do you mean you aren't connected on the tree?
If you want to add a profile for your grandfather and use an approximate birth date, you can do that(once you become a full member). Wikitree is, however, about researching information on your ancestors and trying your best to find sources other than the family's first-hand knowledge.
For more information, see the Honor code. It would be nice if you could read it and sign it once you understand it.
After you are done signing it, feel free to message me if you have questions about wikitree. I'll then answer them and confirm you as a volunteer on wikitree, giving you the ability to add profiles.
