Earlier today, Microsoft announced that Add-ins are coming to its Outlook mobile applications today on iOS, with Android support coming later. In the announcement, there was no mention of the Windows 10 apps; however, Microsoft has confirmed to Neowin that the feature will indeed be coming to the company's mobile platform, saying the following:
Currently, add-ins are available to Outlook users on the web - Windows, Mac (in Insiders). Now, it’s available on mobile for Outlook on iOS, coming soon to Android. It will eventually roll out to Windows mobile, but we don’t have any specifics on timeline to share at this time.
As you can see from the statement, there is no timeline for when this will happen. Presumably, it shouldn't take too long. At Microsoft's Ignite 2016 conference, the company told us that not only does it plan to keep a feature parity between the Outlook apps for Windows, iOS, and Android, but it also plans on adding features for all platforms at the same time.
Obviously, it's going to take some time to get all three platforms on the same path. For example, Focused Inbox is coming to Windows 10 this month, and that's been a hallmark feature of the iOS and Android apps for some time.
Outlook Mail and Calendar for Windows 10 are UWP apps, so presumably, if Add-ins are coming to Mobile, they're coming to the PC version of the apps as well.