Identify the fft points corresponding to 100 Hz, 150 Hz, 200 Hz, 250 Hz, 300 Hz, 350 Hz, 400 Hz. Increase the absolute magnitude at those bin locations as is appropriate for the model you have of the energy contained in each of the harmonics. Note that I say "increase the absolute magnitude", not just "increase the value", as harmonics are not necessarily in phase.
Be sure to also mirror the change at corresponding bins in from the right hand side of the fft: e.g., if the 7th bin from the left is being affected, the 6th last from the end should be affected to the same degree.
To avoid dampening the overall result, increase the value of the first bin (the DC frequency) by the total energy you added to the system.
