Hi Jan,
Did you manage to bring .hgt data in ArcGIS 10? . I could bring it, Add Data > xx.hgt , that's it. maybe i am using ArcGIS 10.2.2. Try updating your lic.
If you need to group your input raster based on some elevation then use "calculate statics" available in Data Management > Raster > Raster Properties .. Don't ask me the technical reason behind this, sorry..
I know its too late to answer you, but still worth sharing my experience for newcomers..
Regards,
Vijai Lal