Hello, we mentioned that in one of the updates, but the beta will be Steam-only. They will also be a separate application, so it doesn't matter whether you already own SCE on Steam or the standalone version - the beta will not change your current install. This means that if you currently do not have a Steam account, you'll need to set one up.
The downside is that you'll have to download the whole game again, of course, but unfortunately there's no way around that - it's much safer this way, given that people tend to mod their games, etc., and we need to ensure that everyone in the beta has a similar experience.