"Can Sources become Witchers?"
Probably, but no one would try.
First, we need to understand what "Source" is: it is a person with a wild, usually mediumistic powers:
A Source,” she explained coldly, “has no control over their skills, no command over them. They are a medium, something like a transmitter. Unknowingly they get in touch with energy, unknowingly they convert it. And when they try to control it, when they strain trying to form the Signs perhaps, nothing comes of it. And nothing will come of it, not just after hundreds of attempts but after thousands. It is one characteristic of a Source. Then, one day, a moment comes when the Source does not exert itself, does not strain, is daydreaming or thinking about cabbage and sausages, playing dice, enjoying themselves in bed with a partner, picking their nose . . . and suddenly something happens. A house goes up in flames. Or sometimes, half a town goes up. Blood of the elves
And uncontrolled powers are indeed dangerous thing:
Uncontrolled powers are an ominous thing. For both the Source and those in their vicinity. The Source can threaten those around them in many ways. But they threaten themselves in only one. Mental illness. Usually catatonia. Ibid
So can source become a witcher? Well, it is like asking "can someone prone to seizures and fits of insanity join the Navy Seals": even when 99% of the time that person is fine, there can be moments when he/she is not in control and then people will die.
There are no examples of source-witchers and most probably no one even tried: the trials and changes that witcher has to go through, would probably even more destabilise the child. Instead, wizards try to contain the sources, so to give them (both the source itself and the other wizards) a fighting chance to control those wild powers.
Just to add a comment to the previous reply: Geral was not a Source. Instead, he was a son of an enchantress, something that is indeed quite rare, because they are usually barren. Probably this gave him the unusual good tolerance to all of the mutations that he went through.