The causes for ADHD are still unknown. Genetics seem to play a huge factor, but environmental and society impacts are still debated. If we did know what causes it down to a biomolecular level, we would be able to make medication that would treat it much better than what we have nowadays. The opposite would also be true - we would know how to make a drug that would induce it, even if only temporarily. So, from a science and medicine point of view, there is no single drug that would product symptoms which would perfectly mimic ADHD.
If you want to really limit your citizens's attention span, focus and capacity to solve long tasks, there are other drugs which you could use. Though you would not mimic ADHD perfectly, you would get those three effects.
Remember: alcohol, weed and most other recreational drugs cause a high that will keep a person from focusing on the here and now. These drugs have also been part of the creative process of many bands. A cocktail with a very light concentration of ethanol, THC and heroine will turn a person into a zombie for a while. Psychonauts who have had frequent contact with DMT and/or peyote are known for being constantly zoned out.
You may also force some conditions in order to get people to lose focus. Episodes where a person has dificulty to concentrate are one of diabete's many symptoms. You would get a lot of other symptoms as well, and a dependency on insuline, but hey, you can't make an omelete without damaging your pancreas.