OK, here's a first proposal for a rewriting of the file db\units\transport_plane.txt to make it into a Zeppelin.
Is the speed meant to be an actual figure in kilometres per hour? (I'm guessing, this being a Swedish game, it won't be miles per hour

) 'Cause that's what I've assumed: early Zeppelins flew at about 80 kph. Likewise, I've given ranges in km. Combat factors: the first model is unarmed, although researching tech 9403 will give it a strat and tac bombing factor.
Code:
## Airship
# 0 - Pre-War unarmed dirigible
model = {
cost = 35
buildtime = 240
defaultorganisation = 30
manpower = 2
maxspeed = 80
surfacedetectioncapability = 3
airdetectioncapability = 1
surfacedefence = 4
airdefence = 4
airattack = 0
strategicattack = 0
tacticalattack = 0
navalattack = 0
transportcapability = 0
range = 4000
supplyconsumption = 3
fuelconsumption = 3
}
# 1 - Basic Airship
model = {
cost = 36
buildtime = 240
defaultorganisation = 35
manpower = 2
maxspeed = 90
surfacedetectioncapability = 3
airdetectioncapability = 1
surfacedefence = 8
airdefence = 6
airattack = 0
strategicattack = 1
tacticalattack = 0
navalattack = 0
transportcapability = 0
range = 5000
supplyconsumption = 3
fuelconsumption = 3
}
# 2 - Improved Airship
model = {
cost = 40
buildtime = 240
defaultorganisation = 40
manpower = 2
maxspeed = 100
surfacedetectioncapability = 3
airdetectioncapability = 1
surfacedefence = 12
airdefence = 8
airattack = 1
strategicattack = 3
tacticalattack = 0
navalattack = 1
transportcapability = 0
range = 6000
supplyconsumption = 3
fuelconsumption = 3
}
# 3 - Advanced Airship
model = {
cost = 44
buildtime = 240
defaultorganisation = 45
manpower = 2
maxspeed = 120
surfacedetectioncapability = 3
airdetectioncapability = 1
surfacedefence = 16
airdefence = 8
airattack = 1
strategicattack = 5
tacticalattack = 0
navalattack = 2
transportcapability = 0
range = 7000
supplyconsumption = 3
fuelconsumption = 3
}