Thanks for the clarifications! I’ve worked for a few non-profit orgs that did not have an official alumni offering, but had informal ones appear. One specific one comes to mind evolved from a happy hour meet up. This org wasn’t the best at management, but did do a nice job of helping employees forge friendships. Some folks left on bad notes, but the large majority remained friends with past coworkers. This led to an alumni happy hour (and current employees managed to sneak in.) The low key approach established an ‘open door’ feel for current employees to interact with alumni.
I suspect a formal program could be tricky, as it could diminish authority of current staff who serve as mentors or are successors to the alumni. An idea is to make the program hyper organized, where employees organize the meetings and set the questions. For instance, a cross-departmental group could monthly fish tank meetings with past employees. This employee group solicits questions from current employees and vets them with the successors to the alumni (to avoid him/her feeling like this is a job performance.) Then, the actual meeting could occur with the alumni, the successor asks a few questions as do some other employees. Of course, a happy hour is beneficial!
Personally speaking, I most value the opportunity to continue relationships in case our paths cross in the future. An incentive then would be to ensure the people they want to impress (CEO, past boss?) are there to keep the relationship going. And any branded schwag is always nice to feel you are part of the crowd.