The regular drill is going to be tougher to set.
As for mixing it, just keep track of the portions. If you want to seed a bushel an acre of oats, wheat whatever, use the weight and add the pounds per acre of grass seed you want.
An air seeder can be calibrated with so many hand turns of the metering system. You turn it the number of turns and weigh the mix that comes out. For 1 bus of wheat (60 pounds) plus the 10 pounds of grass seed, it is 70 pounds of the mixed seed.
A regular seed drill you would have to guess (start with a slightly higher setting than the one for just 60 pounds of wheat) drive a marked distance and refill the drill with weighed buckets. Some drills would be easier than others as the settings are easier to work than others.
My old DK5's are 7 feet each and each one has a highly variable rate set. They seem to change the setting over the course of the same field. I reset them 6 times over 600 acres this year. Needless to say I rent a valmar set up to seed grass.