Short answer: Kinda, but it's not the best tool. Let me explain. Caulk is runnier than filler-more like a thick liquid-and it's meant to seal gaps (like around windows or between tiles) rather than fill holes. A filling knife can spread caulk, but it's tricky to get a smooth, even line.
I tried using my filling knife to caulk around my bathroom sink once. I squeezed the caulk tube, then used the knife to spread it… and ended up with caulk all over the counter, the sink, even my hands. It was a disaster! The problem is, filling knives don't have the small, rounded edges that caulk guns or caulk smoothing tools have-those are made to create a neat bead of caulk that doesn't leak.
That said, if you're in a pinch (like you forgot your caulk tool and need to fix a tiny gap), a small filling knife (1 to 2 inches) can work. Just dampen the blade first so the caulk doesn't stick, then spread it in a thin, even line. Wipe off excess with a damp rag right away. But for most jobs? Grab a caulk gun and a smoothing tool. It'll save you time, frustration, and a lot of cleanup. I learned that the hard way-my bathroom counter still has a tiny caulk stain to prove it.
