Is there a way to create a symbol from spice file ?

xschem symbol files are text files. You could edit it manualy. Easier than point and clicking.

But is there a doc of the format ?

https://github.com/lhrodovalho/sky130_rodovalho_ip__lpopamp/blob/main/xschem/lpopamp_slice.sym Something like this

K {type=subcircuit
format="@name @@im @@ip @@o @@avdd @@avss @@en @@enb @@bpa @@bpb @@bnb @@bna @@xp @@ypp @@ypm @@zpp @@zpm @@xn @@ynp @@ynm @@znp @@znm @symname"
template="name=X1"}

I also found https://xschem.sourceforge.io/stefan/xschem_man/tutorial_use_existing_subckt.html with some info