Add harmonics to a FFT decomposed signal i would like to add even harmonics from the data of FFT of a signal.

How do i do that ? do i have to search for higher magnitude frequencies then add the $2f$ then $4f$ etc ?

I understand how to do it from a sin wave for example but not from a noisier signal.
For each added harmonic, for each bin number below N/2, you could divide by the harmonic ratio, then use that resulting number to interpolate a point below in the FFT, unless the division results in an integer, in which case you could just use the value from a lower FFT bin.  Use a high-quality interpolator, such as a windowed Sinc or other polyphase upsampling filter kernel.  For a strictly real result from an IFFT, make sure the values above N/2 in your harmonic frequency synthesis remain conjugate symmetric with those below N/2.