I don't know what simulator you use.But "length" parameter exists in W-element of HSPICE which is absolutely industrial standard transmission line model.
Generally Nport is not converted to RLGC model.
See
http://www.designers-guide.org/Forum/YaBB.pl?num=1237187544Length information is used for generating RLGC model efficiently from S-parameter in W-element of HSPICE.
[S Model form of W-element]
Wxxx in1 <in2 <...inx>> refin out1 <out2 <...outx>>
+ refout <Smodel=modelname> N=val L=val
[Parameter Description]
Wxxx : Lossy (W-element) transmission line element name.
inx : Signal input node for xth transmission line (in1 is required).
refin : Ground reference for input signal
outx : Signal output node for the xth transmission line
(each input port must have a corresponding output port).
refout : Ground reference for output signal.
Smodel : S Model name reference, which contains the S-parameters of the transmission lines.
N : Number of conductors (excluding the reference conductor).
L : Physical length of the transmission line, in units of meters.