As far as I understand, Cryptainer does also just create a vault. But I might be wrong as I do not use this software.
In generally: You can not migrate encryption technics from one software (cryptainer) to another (cryptomator). So if you want to switch your encryption software, you allways have to decrypt your files with the old software and encrypt them again with your new software.
Means: You have to have access to an unencrypted version of your file and store that into your cryyptomator vault.
If my assumption is right, and your word file actually is stored in a Cryptainer vault, then open this vault to access the word file, open the cryptomator vault, and copy/move the word file from the cryptainer vault into the cryptomator vault.
Please keep in mind that cryptomator isn’t a backup solution. It is designed for storing sensitive data in online storages.