The Adafruit motor controller hat can control two stepper motors, or control forward and reverse on four dc motors, such as the peristaltic pumps in Hello Drinkbot.
But we don’t normally need forward and reverse, so we can take advantage of that fact to control eight peristaltic pumps with a HAT which can normally control four.
To double the pump capacity connect one side of your first motor to the first of the M1 pins and the other side of the motor to ground, and connect your second motor to the second pin of M1, and ground (and repeat for M2, M3, and M4).
Then in the software move ‘FORWARD’ to control motor 1, and ‘BACKWARD’ to control motor 2.
The downside is that this means that you can’t run those two ingredients at the same time. So perhaps we could put non-rivalrous ingredients on each motor controller. So, for example, Gin could be on M1 forward, and Vodka on M1 reverse, under the theory that one would be less likely to dispense gin and vodka in the same cocktail (though a quick google search finds gin and vodka cocktails, perhaps a better case is cream and soy milk for normal versus Hippie White Russians).
In any event, you can now add four ingredients to the stock Hello Drinkbot for just the price of the pumps and tubing. Yay for cocktail ingredient multiplicity!