ANSWER - Funnily enough I was discussing this the other day with a witch who is a close friend of mine - I was trying to persuade her to wrote an article for the site and she said she would but her fundamental belief that a witch is born and not made differs very much from my belief that a witch is someone who chooses to practise - and she didn't want to offend the readers. This serves to prove what I always say - it is rare to find witches agreeing on anything! This topic in particular always attracts strong opinions on either side of the fence.
I think it is important to present a balanced viewpoint rather than just sharing my own viewpoint and so I do acknowledge that there are a lot of witches who hold the belief that in order to consider yourself a witch you do have to have witchcraft in your bloodline. This view however is a tricky one to uphold as many people may have witchcraft further back in their lineage than they are able to trace. There is also the problem that there may have been "natural witches" in the bloodline who refused to acknowledge their craft or who kept it hidden. So the difficulty is that even if you do believe a witch must be born and not made it is hard to know for certain if you do have a history of witchcraft in your family tree.
There is another school of thought that witchcraft is not necessarily hereditary but an innate gift that some are born with and that those not born with a such a gift cannot "become" a witch. Again this view is hard to prove. Who decides how the gift of witchcraft should manifest itself? There is no simple test to determine the nature of a witch. Anybody could make claim to being born a natural witch and it would be hard to prove they were wrong, lying or deluded.
My own view is that "witch" is a descriptor not of what a person is but what a person does.I see a witch as simply being somebody who practises witchcraft. This view has been established from my own experiences of knowing both "natural" and self taught witches and seeing very little difference in the ability, skills and dedication between the two types.
So no - I do not believe you do have too be born a witch too be a witch. I think you need to understand what witchcraft is and commit to the study and practise of it but to my mind that is all that is necessary to become a witch.
Similar questions asked here and here which may illuminate the issue further for you