If you’re in the way to hire a freelance, ask him that question before to decide to work with him:
We need to add few features to a public plugin ([here the plugin name]). Is it possible and safe to customize a public plugin?
He should respond:
- if the plugin provides point of extensions (WordPress actions and filters) I can write a plugin which enhance the public one
- if the plugin DOES not provide point of extension I need to change the code of the plugin, playing directly on the plugin files inside the plugin folder BUT you MUST NEVER update the plugin since WordPress delete everything before installing a new version (and my customization will be lost)
Why a post for a so obvious thing?
Because there are people which come to me desperate because they lost a custom theme, paid to someone, after a Newsletter plugin update. The developer created the theme adding or changing files of Newsletter. Of course on update everything is reset.
A developer who works with WordPress MUST know that behavior and work in a different way. WordPress has the concept of action and filter, widely used by plugins to let other to extend thair functionalities. Newsletter, for example, is able to search themes in a different folder which is not touched on updated (and documented in the documentation pages).
Please, ask the above question to a developer before starting to work with him or only a backup can save you.