BAKs are typically renamed original files, which means these can be pretty much anything. In cases that your was indeed .arx file, you can restore it back by changing the file extension from BAK to ARX. Except for that, we do not know about any other software capable of bak to arx conversion.
In a word, direct bak to arx conversion is not possible.
So don't expect any so-called bak to arx converter or bak to arx free online conversion tool available.