Check the wire rating of the extension cprd.
If it is 14guage wire or larger(12 guage) you will be fine.
If it is a small low power draw computer 16 guage wire is fine.
Most power bars and UPS I've seen say not to use any extensions or power strips in combination with the UPS. Almost nobody follows the manufacturers' guidance though.
Electrons don't care as long as they have adequate wire gauge and clean contacts to travel through. Manufacturers are simply covering their own asses in case their product gets used in combination with a defective or sub-par product and starts a fire.