Customizing WIM is a complex task.
To do it, run in "Windows PE console" something like this:
Dism /Mount-Wim /WimFile:D:\ABoot\iso\sources\boot.wim /index:1 /MountDir:D:\!
reg load HKLM\my D:\!\windows\system32\config\software
reg add "HKLM\my\Microsoft\Windows NT\CurrentVersion\Fonts" /v "Courier New (TrueType)" /t REG_SZ /d cour.ttf
reg unload HKLM\my
Dism /unmount-Wim /MountDir:d:\! /Commit
Another and an easy way is to add user's script in Boot Disk Creator while preparing bootable media, This script is automatically launched when WinPE is loading.