Catalysis of the reaction: 1,4-alpha-D-glucosyl(n) + 1,6-alpha-D-glucosyl(m) = 1,4-alpha-D-glucosyl(n-1) + 1,6-alpha-D-glucosyl(m+1).
1,4-alpha-D-glucan:1,6-alpha-D-glucan 6-alpha-D-glucosyltransferase activity