@JaeHwanChoi pmml should be standard and be able to use in other systems. Though it seems to have some limitations
A lengthy discussion about the exchange of models between knime and python can be found here:
From my experience the best model format to interchange between knime, python, R and even Spark/BigData systems is H2O.ai MOJO:
KNIME has started to support Sklearn nodes. Though I have not tried to interchange their models with python (via pickle for example):
This seem to be the only change since we last had this discussion. The format of a model very much depends on the underlying system and packages. KNIME as a platform is trying to bring them together but I don’t think there is a universal solution or a format to cover everything. For deep learning there is ONNX but my experience here is limited.