You can create a small C or assembly program which performs an AllocAbs call (with location & size parameters), to reserve absolute memory and put it first thing in your startup sequence.
If it succeeds, it blocks the memory you've allocated at the chosen address.
Oh you don't even have to write it as it already exists:
It's generally not possible to do this. Memory boards will add their memory into the OS during power-on using Amiga AutoConfig or a similar mechanism. Changing the memory configuration after boot is difficult if not impossible as the OS and apps may already be using some of that memory.