From the PCGuru FAQ:
1. How can I copy the Kickstart ROM and Workbench disks from my PC to my Amiga?
Kickstart
The Kickstart is a ROM chip contains essential parts of the Operating System and an Amiga cannot boot without it.
A copy of the kickstart can be copied from your Amiga using the TransRom utility which is found in C:\Program Files\Winuae\Amiga Programs folder.
Kickstarts are supplied on the Amiga Forever CD in D:\Emulation\shared\ROM.
Getting a KickStart image using Transrom:
a) Copy TransRom from the Amiga Programs dir that came with Winuae, to a 720k formatted floppy.
b) Insert floppy disk into Amiga and copy Transrom into C: directory. (Assuming you have MSH or Crossdos to read PC disk else use a
serial network and transfer it)
c) Assuming you have enough disk space/ram, type Transrom >kick.rom in a Shell to create a 256K or 512k rom image called kick.rom.
d) Copy kick.rom image back to PC floppy disk (or use serial n/w) e.g Copy kick.rom to PC0:
e) Copy kick.rom image to Winuae directory from floppy on your PC!