Can Dragon Slayers teach Dragon Slayer Magic?
According to the Wiki, Dragon Slayer Magic is a form of lost magic. We know that Ultear was able to learn a form of lost magic called "Arc of Time" from Hades.

However, we know that there are currently two ways you can obtain Dragon Slayer Magic:


Learn it from an actual Dragon.
Implement a Dragon Lacrima into your body.


Therefore, I would say yes, this might be a third way to teach someone Dragon Slayer Magic.