For example, I want to emphasize the word しらない by saying shiiiiiranai, しいいいらない seems a little bit weird to me. How do you write this in japanese character, can we also write this in the form of kanji?
Yes, writing it as しいいいらない is okay, but is relatively uncommon. More common ways to write this are:
- しいぃぃらない！ (with small ぃ)
Needless to say, don't use these in business settings or in formal letters.