pycrfsuite : how to recall the saved models for more training

Question

I am using pycrfsuite now. I Know crf training model's saving.

crf_trainer = pycrfsuite.Trainer()
crf_trainer.train('crf.crfsuite')

So, When I want to tag, i use the source.

crf_tagger = pycrfsuite.Tagger()
crf_tagger.open('crf.crfsuite')

But, I don't know how to recall the saved models for more training.