I agree with Ronald, but also as biofeedback practitioner, I am doing neurofeedback trening for ADHD children and for this purpose we use some data basis for QEEG analysis. There are few data basis such as Neuroguide, Skill... But, everything depends of purpose of your work...I hope that I helped you, somehow...
Our recent study quantifies the "Consistency of EEG source localization and connectivity estimates" across several forward models (BEM, FEM, SHE), inverse methods (LCMV, eLORETA and WMNE) and toolboxes (Fieldtrip, Brainstorm, Berlin Toolbox).
It depends on your research question. If you are interested in finding some neurophysiological features/correlates of ADHD brain, you can probably do 1) sensor space analysis of using spatio-temporal spectral analysis using different methods including machine learning algorithms, and 2) source space analysis by doing inverse EEG mapping and studying perhaps neural mechanisms via toolboxes aforementioned such as the BrainStorm and/or Fieldtrip.