Dear Jan,
Just to complement Joseph's answer:
For the conductance calculation, this would be very easy to do with a recursive Green's function algorithm:
You need to use the concept of non-uniform leads and apply the generalized Fisher-Lee formula [1] and then you just will need to update one self energy for each new length.
I have done this for scanning gate microscopy simulations where we need to change the position of a scatterer at each step (So larger system at each step).
Kwant uses a different approach to get the conductance and the scattering matrix, so if it is just a matter of time, it is better to keep using kwant than writing a new recursive code.
If you need help with the second approach let me know.
I hope this helps.
Adel