The flow you probably want is to make sure you have magic, nsgspice, netgen, xschem, and gaw (the version owned by Stefan Schippers, the xschem developer). In xschem you can create your schematics, and xschem will generate a simulatable netlist, and the netlist can be imported into magic, although there are no tools to auto-route it; you will get configured device instances and pins but you will need to wire it up yourself. Once it is wired up, you can verify the wiring with LVS using netgen, and you can extract a netlist with parasitics and simulate with ngspice.