Yes, you can do that following the pattern of the linear regression fitting with little modification. In linear regression, we fit the equation
a*x+b = y
and in MATLAB we write it as
[x_vector ones(size(x_vector))]\y_vector
to get a and b. But since you already know slope a, your equation become
b = y-a*x
so in MATLAB use
ones(size(x_vector))\[y_vector-a*x_vector]
it will give you the value of b which minimize the least square error.
