I am moving my range. I need an extra 4 feet. It is very difficult to run new wire. It is 8 gauge aluminum wire (says on sleeve 2-8). Can I connect 6 gauge aluminum to the 8 gauge aluminum in a junction box for the extra 4 feet?
Yeah, your plan to splice in a junction box in the basement is fine. The junction box will need 32 cubic inches, so I'd consider a 4-11/16" square box as a minimum. JRaef's idea of a Polaris connector is the right concept, but you'll be better off with a "mini-Polaris" such as a MAC Block.
Upsizing to #6 wire for the last 4 feet buys you nothing. You can just stay with 8 AWG aluminum.
8-2 cable is also not legal for ranges (unless it is SEU cable with a ground in individual strands that spiral around the outside of the conductors. There have been a fair number of incidents of installers using 8-2+ground cable illegally. These cables have a single ground wire that is just a normal stranded wire like the conductors.
Too less power/AWG wiring for the kitchen range seems to be a standard problem.
Just to ignore the code could end in serious problems (insurance, law).
It could be solved by operating a low power range, running new wires/panels, using gas as proposed by Harper or by switching to 2 separate devices, i.e. a combination (hot air/ grill / microwave) oven and induction plates. The last solution does have many advantages.
More details are here: need-a-new-range-but-only-have-30-amps