Creating A Calculated Field Dynamically

I use the Fields Editor a lot when I need to create a new field
that will be a calculated field (such as FullName := FirstName + ' ' +
LastName).  How do I go about creating a new calculated field dynamically?

I am embedding a TQuery in a component and want to create a new field
that will be calculated in the OnCalcFields event of this TQuery.

Any ideas?

Thanks,
Rich