I'd like to know if I were to add additional drives to an existing Raid 10 if this would increase the speed of the entire array?
Generally this will increase the potential read and write capabilities of the system. Some of this depends on your application, though. Also, you want the array expansion to redistribute data across the additional disks in order to leverage the higher number of spindles.
Try this Raid performance calculator and you will get the answser :-)
And yes, it will normaly increase performance.