SO2 ---------------> ? K2Cr2O7?

SO₂ cannot be used to make K₂Cr₂O₇. However, SO₂ can react with acidified K₂Cr₂O₇ because the former is a reducing agent while the latter is an oxidizing agent. The equation of the reaction is:
Cr₂O₇²⁻(aq) + 3SO₂(g) + 2H⁺(aq) → 2Cr³⁺(aq) + 3SO₄²⁻(aq) + H₂O(l)

The acidified orange potassium dichromate solution turns green. This is because orange dichromate ion is reduced to green chromium(III) ion, while sulfur dioxide is oxidized to sulfate ion.