You could try to do it by combining your original US fw boot.bin with the fw_all.bin from 40/50 SG/MY fw. You would be taking some risk and if you don't know what to do, then don't try it. You need to remove, if present, the GUPDATE.GCD file from the .system folder of your unit, and also probably need to remove US Keyboard, Text & Voice folders from your unit before flashing as they maybe need to be replaced with the specific Asian versions. If the US ones are not compatible they may prevent your unit from booting after flashing.
I have not tried this, but warn that it may not be worth the trouble. Even if it's successful to enable SG/MY JCV, there may be other downsides (there usually are some wrinkles when you flash Asian fw to a US/EU device in my experience). Be aware that it is entirely possible to brick your unit if you make a mistake doing this. You should do not only a full files backup but most importantly do an NV backup.
PS: I moved your posts here as they were OT in the [Only registered and activated users can see links. ] thread.
EDIT: It has since been made possible to use Asian JCV files in some US/EU units including x0/x2 units such as nuvi 50: [Only registered and activated users can see links. ]. Big thanks to syzygy for his great tool.