It is not, in fact, a legal routing for any current fare filings. It may have been in the past, but it is not true now. See routing rules below from EF. Given that it's not a legal routing, the only way to book it on a single ticket is with a broken fare. But DL won't let you book broken fares through hubs where they do not have a fare filing unless you have a connection longer than 4 hours to create a true fare break.
MAP CONSTRUCTED LEFT TO RIGHT AND RIGHT TO LEFT
1. GRR-MSP/DTT-ALB
I was going to suggest trying to book it using advanced search by fare class and increasing the fare class one step as a time as IMO this sometimes is a way to avoid broken fares and get cheaper ticket prices, but if the ATL connection isn't a legal routing, the only choice would be to do it as multicity with more than four hours in ATL, which seems to increase the price (perhaps because the cheaper fares don't permit stopovers.
I really miss the functionality of being able to use multicity to construct particular routings without making stopovers along the way.