I've seen it done a couple of ways - one wherein the fuel cell was installed in the right saddlebag, along with a pump that feeds in to the top of the main tank - that way, as you run your main tank down, you switch on the aux pump and re-fill your main tank -
I've also seen a tank mounted in the passenger seat, for gravity feed to the main tank, with a ball valve to turn the flow on/off.
I've also seen flat-ish fuel cells mounted below the top box (making brackets to fit) -
So yes - it can be done and done safely -
You may want to check out goldwingfacts.com - it's a treasure trove of information.
ETA: Fuel capacity is 5.8 gallons (22 liters) for your bike - adding a 20 or 21 liter fuel cell should be very do-able and will bring you within spittin' distance of the 11.5 gallon capacity (44.53 liters)
As I would routinely get 200-220 miles to the thank on my gl1200A - that's about the capacity you're looking for -