Can you walk on a hairline knee fracture?

Possibly: This will depend on the exact location and extent of the fracture, but yes, for many hairline fractures, it is safe to weight bear, especially if you are being treated in a brace.

Bracing: Depends on the fracture level, completeness, medical history or complicating factors, etc. However, most likely you'll need a brace to ambulate for stability.

Usually if braced.: Lots of unknowns make this question difficult to answer. If the fracture is not displaced and does not extend to the weight bearing joint surface you can usually bear weight if braced. Fractures extending into the weight bearing joint surface usually need to be protected against weight bearing. It is usually ok to walk - if braced - with a non displaced fracture of the patella.