I don't think we have the capability to add damage to a custom ability currently. It's on my list of something I'd like to get in, but as with all things it is heavily dependent on a lot of other factors (mainly books in the pipeline, bug fixes, other projects, etc).
For the moment though, what you could do is find a similar ability to what you want from the Offensive Abilities table and customize that. There's some further expansion I'd like to do to the customization options there, but currently you can alter damage dice count/size, DC, range, and description.
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