There was talk about putting a docking ring on the bottom, but I think it never got added.
There has been a lot of recent work on ideas to allow geostationary sats to be refuelled. Also work on support sats that go up and dock with them and take over station keeping duties with their own fuel and thrusters. This can work even for sats not designed for it - the support sat uses the other sats engine bell as the docking target, and essentially uses a barbed probe stuck into the engine bell to make a hard dock.
There has also been work on seeing whether some sats can be refuelled by various other physical means, up to and including drilling holes in their fuel tanks and squirting fuel in. Needless to say it is a lot more hair raising than this. In the middle are designs for refuelling connections explicitly intended for in-orbit refuelling.
However I have not seen any mention that the JWST has any designed in capability to support any life extending missions. Doesn’t mean it doesn’t. But it a very old design, and NASA would have been under immense pressure not to mess with the telescope.
I will bet there will be lots of pressure mounting in a few years to put some effort into scoping out a life extending mission. Something like a support sat that grapples itself on, and then provides some control capability might work. Complicating things is that the JWST is rather autonomous. The distances involved, and a general desire to not have to control its every minute action, means that the telescope runs itself, scheduling tasks, changing direction as needed, and generally getting on with things, without needing outside help. So how it interfaces with a new support system and uses it for general operations is an open question. Not insoluble, but a whole new level of tricky.