How to tell grub.cfg to use isolinux.cfg menu

I've built a custom iso based on Ubuntu 16.04 that boots in both UEFI and CSM (was originally CSM only). I'd like to have grub use the menu in isolinux.cfg. Is there a way to manage this or do I have to build a separate menu in grub.cfg? My isolinux.cfg follows:

PROMPT 0
TIMEOUT 75
DEFAULT menu.c32
F1 help.txt

MENU RESOLUTION 1024 768
MENU TABMSG [F1] help  [Tab] options 
# Dimensions configured for 

MENU TITLE Foxclone

LABEL standard
MENU DEFAULT
MENU LABEL Start ^Foxclone
  KERNEL /casper/vmlinuz
  APPEND boot=casper initrd=/casper/initrd.lz vga=791 quiet splash --
TEXT HELP
   If boot fails, reboot and try
   selecting Safe Mode instead
ENDTEXT

LABEL safe
MENU LABEL ^Safe Mode
  KERNEL /casper/vmlinuz
  APPEND boot=casper initrd=/casper/initrd.lz xforcevesa nomodeset vga=ask toram --
TEXT HELP
   Prompts for a video mode and
   loads the USB image into RAM
ENDTEXT

LABEL check
MENU LABEL ^Check USB for defects
  KERNEL /casper/vmlinuz
  APPEND boot=casper integrity-check initrd=/casper/initrd.lz splash --
TEXT HELP
   Verify integrity of USB drive
ENDTEXT

LABEL memtest
MENU LABEL ^Memory test
  KERNEL /install/memtest
  APPEND -
TEXT HELP
   Check computer memory for errors
ENDTEXT

menu color screen   37;40      #80ffffff #00000000 std
menu color border   30;44      #40000000 #00000000 std
menu color title    1;36;44    #c00090f0 #00000000 std
menu color unsel    37;44      #90ffffff #00000000 std
menu color hotkey   1;37;44    #ffffffff #00000000 std
menu color sel      7;37;40    #e0000000 #20ff8000 all
menu color hotsel   1;7;37;40  #e0400000 #20ff8000 all
menu color disabled 1;30;44    #60cccccc #00000000 std
menu color scrollbar    30;44      #40000000 #00000000 std
menu color tabmsg   31;40      #90ffff00 #00000000 std
menu color cmdmark  1;36;40    #c000ffff #00000000 std
menu color cmdline  37;40      #c0ffffff #00000000 std
menu color pwdborder    30;47      #80ffffff #20ffffff std
menu color pwdheader    31;47      #80ff8080 #20ffffff std
menu color pwdentry 30;47      #80ffffff #20ffffff std
menu color timeout_msg  37;40      #80ffffff #00000000 std
menu color timeout  1;37;40    #c0ffffff #00000000 std
menu color help     37;40      #c0ffffff #00000000 std
menu color msg07    37;40      #90ffffff #00000000 std

Thanks in advance.