The historic way is to store the polynomial coefficients in a dedicated section of the hpb file. Because of the way files are handled in CLIC (see section 3), this is only possible if room for the appropriate section (-27 or -28, see Table 2) was reserved at the time of the hpb creation.
When the data or a smoothed version of it is used, one has to use another solution, which is to store the full bandpass solution (interpolated for all data points, including the high spectral resolution spectral windows if there are any). This is done through option STORE RF /SCAN. This solution can also be used to store a bandpass solution obtained through polynomial fitting. The advantage of that solution is that it can be useful to create an hpb file without the bandpass calibration section(s), which is much smaller in size.