Remember, Microspeak is not necessarily jargon exclusive to Microsoft, but it’s jargon that you need to know if you work at Microsoft.
At the end of a meeting, the meeting organizer might ask if there are any walk-on topics. These are topics that weren’t on the agenda but which an attendee may wish to bring up for the group to discuss.
For me, the mystery is why the adverbial preposition is “on”.
A “walk-in” is a customer who walks into an establishment without an appointment. This sounds like what we have here: A topic that just shows up at the meeting without an appointment (being on the agenda).
A “walk-up” is a service window available to pedestrians, who can just walk up to the window and obtain service.
A “walk-on” is a sports term for a student who joins a school sports team without an invitation. In the United States, some school sports are operated as quasi-professional operations, and schools invite or even recruit students to enroll in their school so that they can be placed on the sports team. But any student who wants to join the sports team can just show up and try to make the team.
My guess is that the metaphor at work here is the sports one: This topic was never invited to the meeting, but it showed up in the hopes that it’s good enough to be added to the agenda.
It seems to me that since you put items on an agenda, that is the most likely reason for the choice of adverbial preposition "on" rather than it being aligned with the sports metaphor. To get on the agenda, it walks onto the agenda. In sports, you walk on to the field. And as you point out the others also follow a similar pattern, walking up to a service window and walking into an establishment....