Could an airplane dock with a set of wing extensions while airborne?
Actually, a similar concept was tested by the USAF in the 50s (like most things in aviation, it seems).  Under the name of Project Tom Tom, the military was looking into the possibility of coupling a parasite fighter (an F-84 in this instance) to a B-36 as a long-distance escort as it flew into the Soviet Union by joining the two aircraft together at the wingtips.  Flying as such, the F-84 would actually power down its engines during coupled flight and could receive fuel from the B-36.  However, the aspect with greater relevance to the question is that the B-36 would actually derive an aerodynamic benefit from this arrangment as well.  In theory, the combined aircraft would also benefit from having an artificially elongated wing as well, and, while I remember reading last year that this actually did result in superior fuel efficiency for the combined aircraft during cruise, I am at a loss to find that source right now.  At any rate, with respect to this question, the B-36 effectively docked with a pair of "wing extensions" (in this case, a pair of F-84s) while in flight and derived a benefit in cruise.  A video of the concept can be found here: 
                
            .  Also, more reading can be found in this text Flying Aircraft Carriers of the USAF: Wing Tip Coupling by Brian Lockett.

Naturally, there were significant downsides to this idea.  While the F-84 actually did manage to engage the coupling mechanism on the B-36 on test flights, logging at least a couple hours of continuous, coupled flight, significant stability and safety issues were uncovered during testing that led to the scrapping of the project.  As noted in a quote from the project summary report:


  The wing-tip coupling mechanism as developed and flight tested in the Tom-Tom program is not considered satisfactory for tactical use and was barely acceptable for test flying. It appeared that a new difficulty and a now shortcoming were uncovered on virtually every test flight. The extent of rework necessary before the coupling mechanisms as designed were acceptable as flight test articles is covered under the DESIGN section of this report.


Similarly, issues with aeroelasticity turned out to be a much larger issue than originally expected (in part because of the change pitching/heaving forces as well as wing mass distribution and stiffness at the joint).  I think that Aurora Flight Sciences ran into similar issues when they were designing an entry for DARPA's Vulture project...the idea was to join multiple vehicles (that were essentially flying wings) to create a super-efficient, high-altitude, solar-powered, surveillance aircraft.  Due to conflicting requirements on wing structure and aircraft weight, I think that ultimately didn't fly...but I don't have any of the details for that.  Just what I remember from a presentation I saw many years ago.

At any rate, I guess that my answer is: yes, it could.  But it is VERY difficult and dangerous to do (at present).