Raja,
Adding a complex component to the tline will not give you loss, for a single frequency you could look at it as a starting angle on a tline of a impedance |a+jb| arg(a+jb) or a tline of characteristic impedance in series with an inductor at a single frequency.
Be careful using this over frequency its even less of what you are looking for, since complex numbers always have a frequency component tied to the complex number. 1/sC or s*L etc.. One way to approximate a lossy tline is to break it into smaller sections where you can approximate the loss by adding a resistor to an AC ground and or a series resistor often as your needs require.
To be even more accurate a lossy Tline has several lossy mechanisms. The three dominant are dc resistance loss due to frequency dependent dielectric constant Er(f). And finally skin effect which is basically a resistor R=Rdc+Rse*sqrt(f/fo) where fo is the frequency where the skin effect starts effecting the resistance.
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