Set Django's FileField to an existing file

just set instance.field.name to the path of your file

e.g.

class Document(models.Model):
    file = FileField(upload_to=get_document_path)
    description = CharField(max_length=100)


doc = Document()
doc.file.name = 'path/to/file'  # must be relative to MEDIA_ROOT
doc.file
<FieldFile: path/to/file>

If you want to do this permanently, you need to create your own FileStorage class

import os
from django.conf import settings
from django.core.files.storage import FileSystemStorage

class MyFileStorage(FileSystemStorage):

    # This method is actually defined in Storage
    def get_available_name(self, name):
        if self.exists(name):
            os.remove(os.path.join(settings.MEDIA_ROOT, name))
        return name # simply returns the name passed

Now in your model, you use your modified MyFileStorage

from mystuff.customs import MyFileStorage

mfs = MyFileStorage()

class SomeModel(model.Model):
   my_file = model.FileField(storage=mfs)

try this (doc):

instance.field.name = <PATH RELATIVE TO MEDIA_ROOT> 
instance.save()