My sister worked for a petroleum wholesaler in Alabama (not going to list the name). They serviced many different brands and haulers, and while they filled each companies truck, requirements for additives were included. Probably not by the driver, with a funnel, or a bucket, LOL...
This was in central AL and they serviced the Shell stations in my town, in the panhandle of Florida. So, they had to include Shell's "snake-oil". She told me this over 10 years ago...now I have to ask to refresh my memory because that's all I remember...
I used to drive a refueling truck at the airport. Some jets require an anti-icing additive because Jet fuel has a density very close to water which allows it to suspend in the fuel. When it gets cold, it clogs filters, which is bad. Many larger jets have fuel heating systems for this reason. The ones that don't, required an additive.
The trucks had an injection system downstream of the tank. It's attached to a 5 gallon hopper that had the fuel additive. When a pilot requested it, I flipped a switch and they were charged like 2 cents more per gallon.
It can be done.