DM gave me a homebrew bow that adds 1d8 piercing and 1d8 poison damage on a critical hit, and regular damage otherwise. Can this be accomplished under create a magical item?

Yes, but the Dice Roller is still too new to combine them into the regular roll. They will simply display next to the weapon’s regular damage as a reminder for you to roll them.

Modifier: Damage->Subtype: [Damage Type]->Dice Count: 1->Die Type: d8->Details: “On a Critical Hit.”

Remember, after saving those modifiers you MUST re-save the entire Homebrew using the main [SAVE CHANGES] button under Basic Information.

Thank you for your assistance, hopefully they will include that feature to the dice rolls in the future.

They plan to. The Dice Roller is still in Beta I believe, and it doesn’t include most of what they plan for it to include.

I will point out however, first they will likely make anything using that Modifier always add damage since that’s how stuff like Flame Tongue weapons work and there are way more of those. After that, they will likely figure out the ]magicitem]vicious weapon;Vicious Weapons[/magicitem], but those only add a Fixed Value, not an additional die roll, and only on a Natural 20, not on any Critical Hit. Since DDB has a contractual obligation to WotC to only implement what the official stuff needs and not necessarily anything else, be warned. Unless there is a single official Magic Item that adds dice to every Crit, and only to Crits, they may not actually implement the system to work the way you want it to.

Anyone know if they still have any plans to update to this? I require a weapon that rolls an extra 1d6 damage on a Natural 20 attack roll, in addition to it's regular 1d8 damage. Similar to how Vicious Weapon work.

If they could do it, then the vicious weapon would work.

Quote from IamSposta >>

If they could do it, then the vicious weapon would work.

 I don't think anyone doubts whether they could do it, but whether they actually want to do it is my question. You mentioned it was in the plans as of February 21, 2021:

Quote from IamSposta >>

They plan to. The Dice Roller is still in Beta I believe, and it doesn’t include most of what they plan for it to include.

 Do you happen to know if they are still planning? Or have they completely given up on the idea of integrating their own content into the Dice Roller?

At this point I don’t think they give a 💩 about the current stuff anymore, and they’re entirely focused on their VTT and the update coming next year. But who knows.🤷‍♂️