What kind of material is this Gear Rack made of ?

Hi, this gear rack is made of plastic.

Can you add this to the kit to have a longer run?

Do you mean to make the sliding distance longer? The length of the rack is usually related to the width of the door, the 1m rack in the standard kit is suitable for doors with a single door width of 1m. If you door is more than 1m, you can buy additional rack.