Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Target
FORGERIO_IMPORT_TARGETParameter: Scale
FORGERIO_SCALE_FACTOR_IMPORTParameter: UVs
FORGERIO_IMPORT_UVWParameter: Vertex Colors
FORGERIO_IMPORT_POLYPAINTParameter: Polygon Groups
FORGERIO_IMPORT_POLYGROUPParameter: Materials
FORGERIO_IMPORT_MATERIALSParameter: Replace Existing
FORGERIO_IMPORT_REPLACE_EXISTING