Calculate input impedance from S-parameters
S11 is in the frequency domain, and the impedance should also be in the frequency domain. If you want to resample the result, simply use the resample command:    

z0           = 50;
A            = importdata("sparameters_lowpass.dat");
N            = 5000;      % number of samples in final result desired

freq         = A(:,1);
realPart     = A(:,2);
imagPart     = A(:,3);

s11_freq     = realPart + imagPart*i; % In frequency domain

z_in         = z0*(1+ (s11_freq))./(1- (s11_freq));

freq_resamp = resample(freq, N)
z_in_resamp = resample(z_in, N)


Note that your formula for S11 would be correct when it is given as a complex signal with a magnitude between 0 and 1 and any phase. Just a word of caution in case the result is in dB magnitude and phase (I don't think that is the case given the extraction of the real and imaginary part shown but that should be confirmed).