Each channel on the Maestro servo controllers has three modes it can be configured in: Servo, Input, or Output. There is not a separate pin available for connecting a button; it should be connected to a channel configured as an input as shown in the "Attaching Servos and Peripherals" section of the Maestro user's guide. That being said, you might be able to still use 6 servos if two of your servos are always moving together (e.g. receiving the same signal) by using something like a Y Splitter Cable to connect them both to a single channel on the Maestro, freeing up a pin to be used as your button input.
Alternatively, you could achieve something similar if you power the Maestro from a switch and configure the Maestro to start the script as soon as it receives power. Then, each time you power the Maestro (e.g. turn the switch off and back on), the Maestro could run through the script once. To do this, you can check the "Run script on startup" option in the "Script" tab of the Maestro Control Center software. Also, if you want the script to only run once, you can terminate it with the QUIT command.