But first, is any way to override common functions to register and login?
Sure is. NTG (which is where I work) has done this commercially for customers. Overriding the existing authentication system is pretty straightforward.
However, Steam uses OAuth and there is already a plugin framework specifically for OAuth providers.
Moreso, there is already a Steam integration plugin AFAIK so all you have to do is install the existing plugin and voila, done.
So to answer the question you asked, yes, companies like us do this and it's not expensive work. But to answer the question you meant to ask, this should already be done for you and you just have to install the plugin.
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