It could when I tested 1 week after the first version of chatGPT was in private beta. It's always been able to convert base64 both ways.
It sometimes gets some of the conversion wrong or converts a related word instead of the word you actually asked it to convert. This strongly suggests that it's the actual LLM doing the conversion (and there's no reason to believe it wouldn't be).
This behavior will likely be replicated in open source LLMs soon.
Edit: ok it looks like it can now convert in base64, I'm sure it couldn't when I tested 2 months ago.