I don't think that will be possible. The protocol for analogue radio was a 20ms packet, consisting of up to 9 pulses and a gap. So there were 10 things in a 20ms packet. The receiver would split each packet into 2ms blocks and send each block to a servo. Any pulse longer than 2ms would overlap the pulse for the next channel.
Obviously with digital radio, it's entirely possible to exceed this limit but I think it's unlikely that there will be an easy, built-in way to do it. Once you've set the end points as high as they'll go, I think that's all you can do.
You could use an Arduino to scale up the pulses you're getting from the receiver to the size you need for the servo. An Arduino Nano is a similar size and price to a servo, and easily able to handle that sort of task.
However I think you'd be better to look at why your servo needs such a long pulse. Presumably it worked OK before you modified it, so it should be able to work from a standard pulse. It's possible that you've damaged it during the modification, or that the resistance of your voltage divider is too different to the original potentiometer, or that you've connected it incorrectly.