I add sounds all the time (hundreds) to my genos. In my opinion most of the free SF2 files are really bad quality.
But if you sample your own VSTi's or synths then the quality can be even better then the onboard sounds (especially when you want those electronic sounds). For instance Leads from the Diva VST transfered to the genos outshines every lead sound in that area (although this is subjective and my own opinion). But also sampling your own EDM sounds from (for instance) a Virus TI or Avenger or Sylenth improves the EDM sound quality of the Genos a lot, i consider the delivered EDM sounds as very weak on the genos (even the Eurodance expansion which has rather outdated and lifeless sounds).
So it depends on what you are looking for, in my opinion the Genos sounds fantastic if you want to play covers from 1950 - 2010, the sounds after that area can be easely added yourself if you want to add those types of sounds. If you are more looking for sounds like guitars or woodwind, etc then don't bother, the quality of those kind of sounds on the Genos is much better then the sounds you will get when you sample them yourself or when you look for old soundfonts.
I use samplerobot and ableton live and cubase for that, but there are some tricks you need to know upfront to get good quality results on the Genos, but there are a quite a few discussions on this board about this topic and i don't really want to upset people with lots of explanations of how to do things, so if you want Bachus i could give you a quick video workshop with all the details and lots of sound examples, just sent me a private message and we will schedule a video meeting (my studio is setup for webinars with multiple cams, so i can quickly show you everything you need to know). This way we can avoid that the people over here get annoyed by the technical discussions and i think you will benefit a lot by seeing and hearing the examples in my studio of what is possible in that area.