SLIC can convert LCIO MCParticles into a Geant4 event by reading from the collection called "MCParticle".
The macro command /generator/filename can be used to specify an input LCIO file.
/generator/filename events.slcio
The -i command line switch functions similarly.
slic -g mygeom.lcdd -i events.slcio -r 1
LCIO event generation is an experimental/alpha-level feature of SLIC. Its primary purpose is to support visualization of previously generated events. It should not be used to generate LCIO output files for analysis.