adding new values to a choice user parameter.

I don't know if I understand your question correctly, but some explanation.

If you create a User Parameter it will be in the list of User Parameters. In this list you can edit it's settings. You can edit the Choice Configuration to add more Choices.

And you can create a new User Parameter. A User Parameter that is a Text Box if you want, or a File Picker.

If you edit your Reader (or FeatureReader, that is more flexible to set up) then you can pick what User Parameter you want to be it's input. So if you create a new User Parameter that is not a Choice Picker but a Text Box you can use the Text Box for the reader.

A Choice Picker will have a set of Choices that you fill in (or Import from a file). And if you set it on Allow Choice Edit, the person running the Workbench can choice to pick one from the list or write one themselves. This will not be appended to the list. If you save all the picks to a file you can Import the Choices ones in a while from this file.

(Or you could create a workbench that reads your FME Workbench file as a text file and change the pick list in this text file. Be aware that you could easily break your Workbench file. And that users can add choices like Bobby Tables...https://xkcd.com/327)