It isn’t possible to produce any tristimulus values for a spectral measurement of a particular illuminant. This is because the ‘stimulus’ in tristimulus is referring to the fact that tristimulus values represent colours as they would be seen by a human observer. And to obtain a tristimulus value, the spectral power distribution of the illuminant is weighted against the 3 human observer curves in the equation used to work out the values. Therefore the illuminant itself can’t actually be converted to any tristimulus values as it is simply the spectral definition of the light source in use. If a human looks at said light source or not, nothing changes, the illuminant is still the same as it ever was.
The only easy way to create a new, custom illuminant file would be to measure said custom illuminant with an i1Pro or similar photospectrometer or better yet, a spectroradiometer.
Lastly, it also isn’t possible to create an SPD file of any Kelvin CCT value as just specifying a Kelvin number value doesn’t provide any info about the actual, physical wavelengths of light which combine to create an SPD for an illuminant. Remember, every Kelvin CCT value (in your case 5080K) is actually representing quite a vast range of observable colours as the K value is but one single point on the Plankian Locus (the curved line on an xy diagram that is used to show approximately where a particular K value might be). It is important to realise that any Kelvin CCT value can always be extended in the direction of the tangent. That’s to say, it can be altered in the vertical direction, either more green or more magenta added to the ‘base’ K value.
I don’t know if that will just confuse you further but hopefully it might go some way to helping you understand why a K value can’t be converted to either a new illuminant SPD file or any form of tristimulus value.
Just out of curiosity, why do you want to change the IT8 file values to 5080K? What purpose would it serve?