We have 2 pendant lights hanging over a breakfast bar in the kitchen. The boxes are easily accessible in the attic. We'd like to move them back a few inches and then add additional wiring for a 3rd box/light. Electrician wants to close/cover the 2 existing boxes, add 3 new ones in a row and use existing line to power the new. Keeping in mind the 2 existing ones could be moved back – creating slack in the line – before adding a few feet for the 3rd/new light, why not just do that? Thanks much. 
Electrical – Is it safe or normal to close/cover junction boxes but use existing line to power new boxes just a few (6-12) inches away from the old ones
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Related Solutions
If there is a junction box where the cable in the conduit connects to the cable not in the conduit, then you can make your connection there. Otherwise, you'll want to run a cable from the existing receptacle to the new location.
If there is a junction not in a box at the end of the conduit, fire your electrician, and find a new one to do future work.
Accessible is a somewhat relative concept. You need to fully remove a recessed fixture from a ceiling to access the connections. This is a bit more difficult than pulling a switch to get at the wires, or even dropping a canopy style fixture, but it does meet the criteria of accessible.
But one of the main rationales for the rule seems to be to ensure a troubleshooter knows where all the connection are, that none are truly buried in the walls where they can't be found.
I don't know if the code has considered your solution, but it sounds like the box you are suggesting is buried, even though it is attached to the recessed fixture. In a sense, you are modifying the fixture in a way not contemplated by the manufacturer. But it might pass muster with some inspectors since it sounds like the junction would be accessible if you removed the can.
There is another way that might suit. The code now allows for certain in-wall splices that can be buried, such as these.
The old wire is inserted into one section of the device, clamped, and screwed in. A new wire is inserted into the other. The halves then snap together and the resultant splice can be buried in a ceiling or wall. If feasible, the completed clamp should be attached to a framing member, but I don't think the code requires it if the wire is left in an unaccessible area.
You could attach this splice to your old wire and run a short extension to your can.
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Best Answer
There's no technical reason that he can't move the boxes. It would certainly be more elegant.
My best guess is that your electrician doesn't like drywall repair/ painting. If the ceiling is textured, that's a strong disincentive, as matching texture is hard.
He'll definitely save you money with his approach.