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.
I can tell u what runFunction() is, I just simplified it for a better understanding.
Because what I just to replace is this condition if( $.inArray( parseInt(ajaxify.data.cid, 10) , CategoriesIds) != -1) instead of going search if the current category is on the array CategoriesIds above (hardcoded), I need to check if the current category is on the selection made by the user on the plugin config page.
And I'm not realizing how I make that connection between the plugin settings and the function.