Assuming A is a row vector and you'd like to add 218 values that extend from the end of A to 0.512,
Aextension = linspace(A(end),0.512, 218);
A = [A, Aextension(2:end)];
If you'd like to specify a fixed interval instead of specifying the number of values to add (218),
A = [A, A(end)+int : int : 0.512];
however, that does not guarantee that the array will end with 0.512.