I want to keep my current Manjaro install but add full disk encryption.
Can I: use Timeshift to back up my current install, reinstall Manjaro & enable full disk encryption, then restore the prev system image via Timeshift?
Hello!
Yes you can. Don’t see any problems there. But you need to consider that timeshift could overwrite configs which are needed for a crypt-setup.
There are a lot of problems actually. If you copy a system which wasn’t configured for encryption into an encrypted volume.
I would advise to copy only you user’s data, then reinstall with encryption and then copy the data back to your home folder.
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And just in case¹ you totally mess up the new install, take a crash-proof backup of your old system first:
Note 1: Unlikely, I know, but still… 
generally timeshift works well with FDE, i’ve no experience with converting to FDE with it though.
I’d guess you’ll need to keep this files from the new install:
/etc/default/grub
/etc/mkinitcpio.conf
/etc/fstab
/etc/crypttab
make sure cryptsetup is installed ( sudo pacman -S cryptsetup )
and after you re-copy those to the restored system you should run sudo mkinitcpio -p and sudo update-grub .
I think that should be all.
As eugen-b said i recommend keeping a separate backup of your /home (i use backintime for this) in case i forgot something 
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Those commands need to be performed in chroot.
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I’ll give this a shot thanks