I don't think it is possible to create a dimension using a calculated field. However, what you could do in Tableau is to create a Parameter of the String type and list down the members you are looking to use. A parameter is different from a calculated field, of course!, but in some cases a Parameter along with Calculated Fields should allow you to get to the result that you are looking for.
Thanks Siraj. To be perfectly honest, I'm not sure I followed this, but as you can see in the other related post, it looks like the proverbial nut has been cracked. Thanks again for the help there.