Caffeine by itself is very very bitter. It's probably a bad idea to add a "shot" of caffeine to any drink. Just add more espresso. It tastes better that way anyway.
Also, a lot of espresso blends contain some percentage of robusta anyway to add color and body. Your espresso blend may already have some robusta in it, thus boosting caffeine content.
Coffee itself, arabica or otherwise, has a sufficient amount of caffeine for the average person. The advent of energy drinks has upped people's expectation of the buzz they will get, but let's face it, these people probably have a problem.
In conclusion, you can always drink more coffee for more caffeine, so there would be no reason to add a shot of caffeine to a drink.
My opinion; For what it's worth.
As a barista, and a coffee advocate, The idea of this offends me. Don't take that the wrong way... I understand and appreciate the need for caffeine and peoples perception of coffee as a convenient means to get it. However as barista's it is our duty to open people's eyes to all that coffee has to offer, and for those people who are looking to get totally jacked on caffeine, I feel like they are abusing the product that is to me the most important part of my life.
My way of dealing with customers like this is to explain to them, in as simple a way as possible, how caffeine affects the body, how caffeine content in coffee works, and why it isn't necessary for them to crack themselves on caffeine.
Try offering an americano instead of a drip and adding an extra shot. Though caffeine content is lower in espresso, it is more easily absorbed and stimulative in the human body. Or try and simply tell them "we're offering a great product, give it a try, and maybe if you start coming here more often for our coffee, your caffeine needs may lower to where is actually healthy" worded appropriately of course
you know what I'm saying?