Thanks for the examples. It demonstrated what I thought.
What you are actually looking for a Lookup Tables. That is, you want to enter a value in one field, then have the software lookup that value in a table, get the related value from that table, and put the result into another field. i.e. Enter "Solti" into the [Conductor] field, have the software lookup "Solti" as the key/index in a Lookup Table, then find the associated value of "Solti, Georg", then write that value to the [conductor sort] field.
Unfortunately, MC doesn't have anything like that, at all. As I said, it isn't really a relational database (people will, and have, argued with me about that), and you can't add extra tables to the database, and certainly can't add Lookup Tables. Whenever this sort of thing has been required by other users, if they really wanted it and the scope was limited, then they wrote large Expressions that had conditions in them and included all possible values. Typically large
Ifelse Expressions. That would mean an Expression that had all possible Conductors, and their associated full names, for example. The scope of your requirements is not limited. Expressions are not a practical solution.
But all is not lost. MC will dynamically generate a drop-down list of all matching values when you start to enter a value into any field. So data entry can be quite fast. For example, if your Library is anything like mine, you have many variations of the "Rock" Genre. If you edit the [Genre] field and start typing "R" you will get a drop-down list of any Genre that includes "R". Continue with "o" and you get anything with "Ro" in it. By the time I type "Rock" I get only Genres in the drop-down list that include the word Rock and it is easy to select the correct value. This has the benefit of standardising data entry, to avoid spelling errors or the creation of similar values in a field. For example, "Heavy Metal" and "Heavy Metal Rock" should probably be considered the same, so only one of those should be used in the Library. The drop-down list can assist with that.
The other benefit of just using the drop-down lists as intended is that you don't need three new custom fields as per your example. It is easy just to use the standard field to enter the full value. If you still wanted to have the abbreviated values in the Library, for use in Views or whatever, you may be able to create a custom field with a standard Expression to create the abbreviation. For example, to take just the Last Name from the [Conductor] field and put it into a [Conductor Surname] field or similar.
That's the best idea I have. I hope if anyone has a better idea they speak up! I'm not too proud to be wrong, or learn new stuff.