My buyer is agreeable to the seller maintaining possession for 30 days after close. Is that something that can be written into the purchase agreement?

While it can be included in an offer to purchase, the West Michigan Regional Forms Committee provides a Temporary Occupancy Addendum that defines the terms of that arrangement and is recommended for use in these instances. That Addendum addresses the length of occupancy, the occupancy fee (if any), maintenance of the property during that time, insurance requirements, failure to vacate at the agreed upon time, notification requirements if early vacancy occurs, and more.