bhyve and freebsd memstick installer

Hello,

context: freebsd-12-current server, amd64

I usually install a freebsd guest like this:

sh /usr/share/examples/bhyve/vmrun.sh -c 2 -m 4096M -t tap0 -d guest.img 
-i -I FreeBSD-installation-dvd1.iso guestname

I only have memstick.img - how do I either:

1. convert the memstick to dvd1.iso
2. make bhyve use the memstick installation iso (I don't think it can do 
this as the option to do so isn't in vmrun.sh)

thanks,
-- 
J.