In short, no, Geneos supports only the following severity levels
The use of the just 4 severity levels is an active design decision on the part of ITRS. The requirement for more levels is normally because an environment has too many alerts which needed to be sorted into further levels to manage effectively. A more practical solution is to review carefully what is alerting, and reduce the frequency, which in turn reduces the need to order large lists.
You may also consider using 'Undefined' as a valid severity, and OK for informational. I.E. everything you monitor by default is undefined, and you may consider that things only go green when they demand attention but not action.