For RAID 0, can you add a drive to an existing setup?

RAID 0 speeds up reads in many circumstances, but it also slows down access time. RAID 0 works best for large file reads and as scratch space for working with large files. It might help your boot time a little bit but I doubt it is going to make that kind of difference where you would say "wow".

As to if you can add a drive to your RAID 0 array that is in place that depends on your controller. The term if called online capacity expansion, but you will need to check with the maker of your controller to see if it is possible.

Just in case you were wondering how much adding drives to the array might speed things up, that most of the time also depends on the controller. Controllers built on motherboards have been known to have a throughput of around 120mb-150mb in some cases (even if they say SATA II). If you have a processor based controller then some of them such as the 5 series Adaptec controllers have a throughput of 1.2GB a second (gigabyte not gigabit). Adding more drives to a motherboard controller will be less beneficial than adding them to a processor based 3rd party controller.