The James Webb Space Telescope (JWST) will be an incredible addition to the scientific community once it is launched and operational but the relative short planned mission time will make this a time critical operation.
The planned minimal mission time is 5 years with a projected 10 year life span of the spacecraft. Those 5 years seam quite short if you consider that the telescope is now delayed for longer than it supposed to be working. Those 5 years are also outshined by the 29 years Hubble has been operational (disregarding the optician consultation Hubble needed).
JWST may be in a considerably more "complicated" orbit than Hubble but launch cost as dropped significantly in the last few years. Therefore the price of a launcher for a planned refueling mission would only be a fraction of the whole JWST project.
Taking all this into account and the fact that Hubble was serviced every few years:
Why are there no plans to refuel JWST and is it even possible?
On the German Wikipedia article about the JWST I found a mention of the possibility of docking to the JWST for refueling.
Während der vorgesehenen Nutzungsdauer sind bisher keine Reparatur- und Wartungsmissionen vorgesehen; es gibt jedoch eine Vorrichtung, die eine nachträgliche Ergänzung der Treibstoffvorräte durch eine Robotermission ermöglichen würde.
Roughly translating to:
In the intended service life no repair and maintenance missions are planned; However, there is a mechanism that would allow for a subsequent supplementation of the fuel supplies by a robot mission.
There is no source for this information so it may well be outdated or outright false, but this gives me the hope that JWST may be refuel-able and will be if the necessary budget gets allocated.
Is the payload adapter for the Ariane 5 maybe useable as a docking mechanism for refueling or could a probe just dock with JWST and be used for station keeping without any fuel transfer?
I know there've been multiple question on the serviceability of JWST for example using Orion as a shuttle replacement, but I do not ask about changing instruments how it was done with Hubble but rather a "simple" reboost/refuel for station keeping.
This question ask about how the JWST will be serviced with the answer saying that it will not be serviced because:
There is currently no servicing capability that can be used for missions orbiting L2, and therefore the Webb mission design does not rely upon a servicing option.
This answers the other question about servicing but not mine about the refueling capabilities as it is something completely different to send a shuttle-like mission to JWST for servicing compared to sending something like an ATV launched from a Falcon 9/Ariane 6 for a refuel/reboost.
I just can't imagine that another 200 million USD for a launcher and automated probe wouldn't be justifiable if a spending of 2% of the overall project cost could mean a doubling of the life span.