@DarioBa
Hi and welcome to the Forum
Full-Disk encryption involves a lot more as just adjusting files into another format, that’s why a complete reinstall is the preferred (and most reliable) way to do what you need.
Trust me it’s way less work and error-prone to just make external backup of your home dir and commutations of your system and restore them from the backup after you have reinstalled your system on a full-disk encrypted disk.
- Make an External backup of your home directory.
- Make an External backup, apart from above, of your system customizations.
- Do a full fresh install of Manjaro with full-disk encryption, see the tutorial below, without using anything from your backups yet.
- If all works as disired, ONLY THEN, consolodate your backups to modify your system acordingly.
Do NOT do plain restore from backup as files for your system customizations, because that will render your system inoperatable !
For some info on what all is involved and a way to boot into it without password, see this tutorial i made some while ago: