It's a bit more complicated than that. There's really no good way to do this without the facility for it being baked into the dataset itself. I wrote a blog post on it a few years back, exploring the topic in detail:

http://tech.turbu-rpg.com/122/adding-non-data-fields-to-a-client-dataset

It's about TClientDataset, but the principles involved can be applied to other datasets.

--- In IBObjects@yahoogroups.com, "IBO Support List" <supportlist@...> wrote:
>
> You have to use the mechanisms that are provided with the TDataset and
> TField. I have not ever done this before at runtime only but I think is all
> you do is create an instance of the TField. Please refer to the help files
> for Delphi on how this is done.
>
> Jason
>
> -----Original Message-----
> From: IBObjects@yahoogroups.com [mailto:IBObjects@yahoogroups.com] On Behalf
> Of patrick_marten
> Sent: 03 December 2012 05:47 AM
> To: IBObjects@yahoogroups.com
> Subject: [IBO] How to add a calculated field to an IBOQuery at runtime?
>
> Hello,
>
> an IB_Cursor offers the method .CalculatedFields.Add for that, an IBOQuery
> doesn't.
>
> Is it possible to add a calculated field to an IBOQuery at runtime?
>
>
>
> ------------------------------------
>
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