pdg id 1000022 to refer to some new dark matter particle
I think the issue is that the neutralino (1000022) is defined to be a Majorana fermion,
whereas it is treated as a Dirac fermion in the LHE file.
In that case, there would be 3 solutions:
1. use sed on your file to change -1000022 to 1000022 (messy)
2. use sed to change 1000022 to, say, 18 (a 4th gen neutrino).
3. redefine the neutralino in your Pythia8 config to be a fermion, i.e.
add the following line to your config file:
'1000022:all = chidm chidm_bar 2 0 0 10.0 0.0 0.0 10000.0 0.00',
(the actual mass should not matter, because it is updated in the SLHA part of
the LHE file)