There's an option titled "Allow editing of value list" but it can only be used with custom values where the values are actually typed into the value list definition.
What you can do, is place a button next to the value list for editing your value list. This button can perform a script that adds a new record in the table from which your values are drawn with the needed category and value entries. It can collect the values via a Custom Dialog, a Floating Window Created with the New Window command or it can just switch layouts for you to enter new values and then switch back.
Thank you very much, I used a Custom Dialogue and it works perfectly!