One way to do this:
You will need two hammer head objects. One of them is simulated as fluid, one of them is not. I will call these the "hammer fluid" and "hammer static" objects for short.
Your "hammer fluid" object should be located in the position where you want the fluid simulation to start (i.e., the first frame where the hammer head touches an object / the floor). For convenience let's call this Frame 100. Using keyframes, the viewport and render visibility of the hammer fluid object should be set to OFF from frames 1-99, then ON starting at frame 100. Once you have added the object to your fluid sim (as type "Fluid"), you will want to give it an "Initial Velocity" that is appropriate to whatever the hammer head was doing at the moment (e.g. falling straight down, moving side to side, etc.).
Your "hammer static" object should be visible from frames 1-99, then invisible starting at frame 100, again using keyframes.
From the viewer's perspective, the object will seamlessly transition from the static object to the dynamic object on frame 100.
In terms of the fluid domain, obviously it should be given the fluid material. It should also be invisible from frames 1-99, visible starting on frame 100. You would give the simulation an "offset" (Physics tab, Fluid section, under Time) equal to the transition frame times -1. So in this case the "offset" would be -100.
Quick tip: you won't be able to bake your fluid sim unless you're on a frame where the fluid domain object is visible!