The ability to add a virtual column to an existing model outside of the modeling tools is not yet supported except with specific items like dates , outliers, clustering etc. We plan on adding this in the future and it will allow you to do this and more. (Although creating a duplicate dimension is somewhat redundant unless it includes some manipulation or calculation).

Unlike Tableau, where your model is local and specific to the project (and you), Pyramid has to ensure we come up with a technique that handles the shared model construct on a shared platform that may be used and accessed by multiple users in multiple projects - which makes the mechanics significantly more complex. Never the less, its on the agenda.