Manually tell it to dump its fuel at the colony you want (make sure the design is checked as a tanker), then tell it to sit over a Gas Giant that has Sorium in it. When you receive the message "fuel tanks full", repeat. You can save the order so you can issue it in one click.

And on a different note: Fuel Harvesters don't need Cargo Space. So you can downsize by ripping Cargo Holds/Handling out, unless you need them for some sort of ultra long range transporters.

Hello to everyone!
As a noob player, I'm currently facing a big fuel shortage in my empire.  I do have some colony with sorium, but I would like to expand my harvesting operations by using the 44 milion tons of Sorium located at Saturn. 
I've built a little Fuel Harvester just to learn to use it, here it is:

Nero class Fuel Harvester    100 000 tons     390 Crew     2222 BP      TCS 2000  TH 1500  EM 0
750 km/s     Armour 1-191     Shields 0-0     Sensors 1/1/0/0     Damage Control Rating 1     PPV 0
MSP 14    Max Repair 75 MSP
Intended Deployment Time: 120 months    Spare Berths 0   
Cargo 25000    Cargo Handling Multiplier 25   
Fuel Harvester: 20 modules producing 1120000 litres per annum

Aquila Turbines 300 EP Commercial Ion Drive (5)    Power 300    Fuel Use 7. 96%    Signature 300    Exp 5%
Fuel Capacity 8 750 000 Litres    Range 197. 9 billion km   (3053 days at full power)

This design is classed as a Commercial Vessel for maintenance purposes

Now, the problem is: both in the wiki and in some post here in the forum people says that, to use this, you have to move it to a gas giant with Sorium, then activate the conditional orders
- Fuel Tanks Full
- Unload Fuel at Colony and move to Sorium Gas Giant

I tryed, both by parking it near sarurn THEN activating the orders, and by activating the orders BEFORE leaving earth, but in every case the harvester just move to saturn and simply stays there, giving the message "Harvester fuel tanks 90% full".

What am I doing wrong here?