More or less the title, was wondering if it was even possible to do so. I've made copies of pre-existing divine domains to see how, but couldn't find that specific feature formatted in those builds. Any help is appreciated.

At the bottom of the main page, where you have the description and the name, you should see a box that says "Additional Specific Spells". By entering the spells into that box, they should appear in the character sheet as prepared spells. The actual Domain Spell feature should not have any spell connected to it.

I am aware of that, what I was asking for in particular is how to emulate the actual “Domain Spells” class feature blurb that appears in the character builder. Specifically the one that lists the domain spells in a chart.

There are two ways. The easiest way is to create a homebrew subclass using a template of an existing subclass and just edit the subclass specific spells in the table there, oooor you edit the source code (there's a button in the editor's header for doing so) and paste into it the html table stuff copied from the source code of an already existing table. 

It might seem weird that I am responding to this question from 2019, but this question was the only thing I was able to find when I googled for "how to add domain spells to homebrew subclass dnd beyond", so if that happens to someone else too, here is the answer ^^

The other answer is to simply drag-highlight the table from a subclass and copy/paste it into your own feature.

Ok... I'm taking a party through Baldur's Gate and I want the Cleric to be a Bhaalspawn. At lvl 5 I want them to have vampiric touch as a domain ability because of this... how do I add this to their sheet?

You could do it a number of ways. The easiest would probably be to do it as a feat and just give it to them when they hit 5th level. Otherwise you would have to homebrew their subclass for them and add it as a feature.