[BioPython] add models to a structure

[Ramon Crehuet]

Hi all,
    I would like to convert two structures created from different PDB 
files through the PDBparser into two models of a new structure. Or 
alternatively, add one of the structures as another model (a new child) 
of the first structute instance.
Being s1 and s2 structures, I tried:
s1.add(s2[0])
but this doesn't work, probably because both model id's are the same 
(=0). How can this model id be changed?
    The question may be very naive, but it is more general. How can I 
convert a group of chains into a new model or a new structure or vice-versa?
Thanks in advance.
Cheers,

Ramon