I am not sure if this function was available at the time you asked the question, but at this time you can find the following references in the assimp documentation:
Assimp::Importer Class Reference
const aiScene * ApplyPostProcessing (unsigned int pFlags)
// Apply post-processing to an already-imported scene.
There is also a C-API style function in cimport.h
cimport.h File Reference
ASSIMP_API const aiScene * aiApplyPostProcessing (const aiScene *pScene, unsigned int pFlags)
//Apply post-processing to an already-imported scene.