Hi Mark, unfortunately we can't create custom arps in Genos.
Having said that, usually I use multipads that can act as custom arps to some extent at least: you can start them synchronously via registration memory and they have an associated voice and can react to chord changes in the left hand. Its possible to create pretty complicated multi pads with proper programming skills (probably better to use computer because the multipad creator that genos has onboard has a limited functionality)