@Brix You would need to edit (yourself or with someone's help) the /includes/view-functions.php file which contains the code for rendering the Machform fields. You'll see there are different functions for each field type.
For example, for rendering dropdowns, you would customize the "function mf_display_select(){".
Just keep in mind that this will in fact be a global change, so if you hard code values here that overrides the final field render at the end of the function, this will impact all dropdown fields you might leverage on any/all forms and completely ignore the values you set in the form builder interface.
Thus, you would probably want a mechanism to skip this new custom sequence if, for example on one of your forms, you did in fact want to create a drop-down with custom values and not the global values you defined.
You could achieve this likely by leveraging the "Custom CSS Class" property of the Machform field (editable in the form builder when focused on any given field.)
While the obvious utility of the "Custom CSS Class" property for Machform fields is so you can customize it visually/on the front-end...this value is something you can also leverage on the back-end, to test for, as a flag to whether or not you want this globally defined list to be used. Then in the field render sequence of the mf_display_select() function, you could add a check for the special value in the "Custom CSS Class" property, for example "globalDD".
Then in the field render section, if( $custom_css_class == "globalDD") { / render global Dropdown, ignore default render / } else { / default field render / }
To recap, edit view-functions.php. Possibly set things up so your dropdown fields have to have the "Custom CSS Class" property set to a specific value so your back-end code can check for that flag and if "true" then you can render your custom, global dropdown values. Allows you to opt-into this global DD for any given field (instead of just hard-coding it and preventing you from doing custom dropdowns when you actually want custom values for other fields.)
At least this is one way to do it, not perfect but something that came to mind.
Hope this is helpful.