I'm creating a level 20 character for a 1-shot, and chose the Rod of Lordly Might as my main weapon, using it mostly as a Flame Tongue Rapier. Is there an option for activating it so that I can roll in DDB for the correct damage? 

I tried just adding a Flame Tongue Rapier to my inventory separately, and that does show up with rolling options for attack and damage, but not for flame damage.

Due to its variable nature they can't code it all into one item.  Adding a separate Flame Tongue rapier was the correct way to do it.

For the flame damage they still don't have a good way to stack multiple damage types on one weapon, so you have to roll the 2d6 fire damage using the custom dice roller in the bottom left corner.

Thanks @Gruntler.  I figured that was likely the answer I'd get, but thought I should check.

You can use custom actions to make it easier to roll the extra damage.

Thank very much for that! I only just now noticed my notification that you had posted a reply. I never knew you could do those custom actions until now!