Legitimately? No. Using the Arch forum requires you to have installed your Arch Linux system the Arch WayTM. Even using an install script disqualifies you from posting there.
However, if you want to be deceitful (I would never recommend such a thing, of course!), you don’t need to convert anything, as long as you’re mindful of what you say (you wouldn’t share a screenshot with neofetch displaying a big fat EndeavourOS logo, of course). It is important to know your system well enough not to accidentally reveal the fact you’re a liar. If they catch you, you’ll probably be banned, but if you behave well they have no reason to investigate you… Just be mindful, for example, that you do not ask about some error and it is revealed that an EndeavourOS package is causing it – that wouldn’t be tolerated (and rightfully so, you’d just be wasting everyone’s time on the Arch forum).
But in practice, there is no way to tell the difference between an EndeavourOS and a vanilla Arch system to which you’ve just added a tiny third-party repo. If you just want to get rid of the EndeavourOS branding and preserve all the functionality (like keep using EndeavourOS repo), you can simply remove the eos-hooks package and reinstall filesystem and lsb-release.
That said, you should try installing Arch. It’s not at all difficult, it’s a nice, intermediate-difficulty exercise in following a guide… And then when you say “BTW, I use Arch”, there won’t be that tiny part of your soul rotting away, because deep down inside you know that what you’ve said was a lie. Not that anyone else cares, but for your peace of mind… 