Is it possible to script a button that takes basic text input from a field and adds it to a drop down list? I'm sure it is but I lack the skills to do this...your help is really appreciated.

I'm sure it is but I lack the skills to do this

The important lesson here is that a field in order to build a value list needs to rely on stored values, this means that as soon as related values, summaries and global fields comes in the way will the drop down stop to work ... except if you use a local field in which you have used the lookup function to grab related values.

You would face a hen/egg problem with the transactions likely to be the data that generates the drop down, which need to exploit event trigger plugins that via script fills the field(s) behind the list with new values every time you have made a selection.

--sd

Why not check the Include "Edit..." layout option for the value list?

Alternatively, if you turn on "Use values from a field", the value list will automatically add whatever unique values are in the field (including entries you may not want, unfortunately).

Alternatively, you could create a table for value lists. You would then need an interface to deal with it.

But what you cannot easily do is have some automated process add specified values to a Custom value list from a field, unless you use something like Quickeys. It would not be easy (or likely safe). How would you tell it to do it, and how would it know what to do?