An AxCrypt-encrypted file is just that – a file. It’s like a Word-file, or Excel-file, or JPG-file or whatever. A file is a file. It’s a container for some information in a format that a specific application can interpret and do something you expect with.
So, yes, you can back them up. Or do just about anything with them that does not modify the content. You can back them up, copy them, send them via email, put in a cloud service or whatever. You can put them inside zip-archives or other archives.
As long as you can restore the file just as it looked after being encrypted by AxCrypt, i.e. unmodified, if you know the password and have AxCrypt you can open it or decrypt it.