Yes, sure.
As soon as I know the name of these packages, these can be - depending on the package either added to documentation and/or be added here https://github.com/Whonix/Whonix/blob/master/build-steps.d/1120_prepare-build-machine for automatic installation during build.
Yay! 
https://github.com/Whonix/Whonix/blob/master/build-steps.d/1120_prepare-build-machine#L236
package Debian -- Details of package syslinux in buster
contains /usr/lib/SYSLINUX/ (all upper case)
Debian -- File list of package syslinux/buster/amd64
Would you like to add it here
https://github.com/Whonix/Whonix/blob/master/build-steps.d/1120_prepare-build-machine#L236
or I could add it there too.
Btw why isolinux / syslinux?
Low priority: worth switching to grub2? Would you know how to do that?
Would grub2 be better for UEFI support?
And would grub2 be better for Secure Boot support?
(enable Linux kernel gpg verification in grub and/or enable Secure Boot by default)
Maybe it doesn’t really matter since if we wanted to be compatible with Secure Boot (on the host) we might be able to combine shim-signed with isolinux/syslinux too?