They are based on game scripts and in order to add more items, you need to write new scripts.
NOTE: The pictures below do only illustrate the default scripts in Crysis. Your list might look different, depending on your script changes.
The item entities can be found in RollupBar > Entity
HitPoints: Specifies the hitpoints of the object
initialSetup: Specifies initial setup perameters
Mounted: Specifies if a gun is mounted
Physics: Specifies if physics is applied to the object
Pickable: Specifies if the object can be picked up
SmartObjectClass: Specifies the smart object type of the object
Useable: Specifies if the object is usable
Respawn: Specifies if the object will respawn once destroyed
Timer: Specifies the time the object takes to respawn
Unique: Specifies if the object is unique Other Item Entity Parameters
Click here for the Miscellaneous Parameters reference guide, here for the Material Layers guide, here for the Miscellaneous Entity Parameters guide, here for the Entity Links guide and here for the Entity Events guide.