When I create a custom weapon I would love to be able to customize the damage please.  

I have a rune knight that has purchased a large size halberd, for when he uses Giant's Might, that does 2d10 damage instead of the 1d10 damage a halberd normally does.  I see that you can overide the To Hit.  I would like to overide the standard damage as well.  I'm sure I can homebrew one but it seems like if one can overide the To Hit one should be able to overide the damage as well.  Thank you for your consideration!

Override to hit and Override to damage both work the same, you keep the die the weapon has and can change the modifiers it uses.

The only way will be homebrew or a manual roll

what if I want to add, say, a d6 of cold to the damage?

You could add a modifier for that damage but it won't let you auto-roll. 

If this weapon is just personal use you can just add a custom action to make it easier to click to roll the damage. Here's how: