VBA coding to create functions in an Excel document

We have a working excel file that is used to within the company, we need to add some functionality to the existing code (already being used) to improve some parts of the document process.

Using the latest version of Excel/VBA, I need to add some buttons/functions that add/remove items from drop down lists that are being call on in other parts of the document. By easily adding/removing items from one list, it adds/removes the option to select that item in another area of the documet.

This update must work alongside the existing code already been developed. We dont want to recreate this again in a different version of excel... you must do it in the latest version.