Current Boosting Techniques for Source Side
This would work if you could tell your laptop "hey, be nice, don't use more than 10W worth of power from the source..."

But your laptop has no such feature, when it is plugged into the charger, it is designed to charge as fast as possible, which means sucking in as much current as it can...

There is no way to do this, as the others said. The simplest is to get a spare battery for your laptop, by the way.