Here is the scope of what we would want the script to do.
Automatically, extract the data in the IPTC caption field of an image and write it to the Alt Text field of Powerpoint when the image is placed in a PPT slide.
Be a droplet that you could drop a PPT file on and have it go through and write the IPTC caption of all the images to ALT Text that has already been placed on the slides in the file all at once.
I already asked this question in the Microsoft forum and this is the response I got. Its not a solution by any stretch but might give you some direction.
Jim Gordon MVP Replied on January 22, 2020
MVP | Volunteer Moderator
Yes, you can automatically import IPTC fields of an image with Microsoft PowerPoint.
There are some prerequisites. First, you need to know both Microsoft Visual Basic for Applications and AppleScript well enough to write the script you will need.
Second, you need an additional application that can be scripted with Applescript and Microsoft Visual Basic for Applications. Graphic Converter is a program that is programmable with AppleScript. Because you can program AppleScript from Visual Basic for Applications and vice-versa, you can control Graphic Converter from within Microsoft Visual Basic for Applications or automate the whole thing in AppleScript and run VBA from within AppleScript. There may be other photo editing applications besides Graphic Converter that are scriptable with AppleScript. Photos is not scriptable. Adobe creative cloud is not scriptable.
Below is a screenshot of Graphic Converter's AppleScript dictionary showing IPTC properties are listed.