Does anybody know how to import the sky130 spice library into qucs-s?

The SKY130A spice library was made for ngspice. As far as I know, qucs is compatible with ngspice, so there is no need to import it. Currently, the design flow uses Xschem for capture and for testbench simulation. Take a look into some of the examples and you may get to simulate on qucs too. https://github.com/efabless/caravel_user_project_analog/tree/main/xschem

My problem is with the software itself, it seems... I'd like to run a simple 2-port Sp simulation (say, to extract the maximum unilateral gain) of a sky130 mosfet in qucs-s, and I'm quite clueless on how to do it

I made a quick search for s-param simulation with ngspice (http://ngspice.sourceforge.net/docs/ngspice-manual.pdf) and it seems it uses the spice AC analysis. Qucs have a different approach. It uses a AC simulation with S-parameter probe (https://qucs-help.readthedocs.io/en/spice4qucs/RF.html). Ngspice has some tools for s parameters extraction but I have no idea how to use it. I recommend you to try to simulate any circuit using the spice model with qucs first. You can start with the FET characterization testbenches from the PDK (https://github.com/google/skywater-pdk-libs-sky130_fd_pr/blob/f62031a1be9aefe902d6[…]6/cells/nfet_01v8/tests/sky130_fd_pr__nfet_01v8__nfet_vth.spice)

S parameters are linear models of the reflection of a n-port device over frequency. The skywater PDK has BSIM models which model the device through differential equations. If you want to simulate s-parameters you need to create a spice simulation with a certain device size (W, L, M, nf) and bias (Ids, Vds, Vbs) to extract the scattering parameters. Once you have these values as a table of text you can create a touchstone file that can be read by Qucs s-parameter file read. It's possible to do all this inside Qucs but you need to use the ngspice (or Xyce) backend to exteact the s-parameters