This is a case of onomatopoeia/擬音語【ぎおんご】 being used as an adverb. かぱん here illustrates the sound of the rice cooker as it opens. If you look it up on Jaded Network, it tells you it is the sound of "something opening". The sentence doesn't exactly sound natural in english, but this is a more literal translation:
I opened the rice cooker with a "kapan" sound.
A lot of onomatopoeic words are used this way and can be used adverbially. Here are some more examples:
It is raining with a "zaa zaa" sound.
Natural translation: It is raining cats and dogs.
To shut with a bang.