Yes you can make new damage types. use damageType: 10 or above (-1 works too)
the new type won't do any damage to units until you can units a resistance value to it however.
but... the weapon will still damage terrain, and it will revert itself to HE damage if you give it a blast radius value.
The UFOpedia will list these new damTypes as "unknown" dunno if you can change that or not.
Uses...
you could give robotic enemies a resistance to this new damType... Creating EMP that only affects robotics.
you could make a weapon that only affects terrain...
Limitations...
can't give it a stun effect or a blast radius.
Example ruleset item included
Edit: yeah... I sorta forgot to define clipSize in that ruleset -_-' it's fixed now.