Hiragana/katakana just represent syllables. So yes, all Kanji can be replaced by hiragana/katakana. Usually it's replaced by hiragana because it's Chinese characters; except in cases of emphasis in text it could be replaced by katakana, or situations when the word is actually of non-Chinese origin (it's not compulsory to write in katakana though).
The thing you have to be careful about is how you replace the Kanji.
Japanese use Kanji often for representation of a meaning, so the same Kanji could be pronounced in various ways, depending on whether it's used as a verb or a noun.
You have to be particually careful for compound kanji verbs. Usually in verb form, for example 検査する (kensa-suru), the compound kanji is pronounced similar to chinese + suru. However there are exceptions when the verb containing a compound Kanji does not end with the suru.
One example I can think of is 狼狽, which has similar meaning in Chinese.
狼狽 can be used by itself, so it would be pronounced Roubai which is similar to Langbei. That's the Kun pronounciation.
狼狽える on the other hand is pronounced as uruta-eru, so the hiragana representation is completely different and the compound kanji uses the On pronounciation.
Also, not all Kanji are pronounced in the Chinese form even if they are nouns. For example, cigarettes, can be written as 煙草 or タバコ but it's pronounced Ta-ba-ko (tabacco). Or like, Velvet which in Japanese is pronounced as Biroudo and can be written as ビロード or 天鵞絨, supposedly originates from Spanish "Velludo". In such cases if you replace the kanji with Japanese characters you usually use Katakana because they're of non-Chinese origin. This is not a hard and fast rule though. For example, Tempura, is usually written as hiragana てんぷら or 天麩羅 yet it's actually a word of Portugese origin.
In summary, the syllable representation of Kanji is a function of when the term was adopted for use in the country. Typically, newer foreign words do not have a Kanji representation. But, old foreign words, even if it does not have a Chinese origin, may have a Chinese representation for it. Note that net speak is actually changing a lot of that. For example if you look at my signature, 自鯖 actually looks like Self-Saba (Mackerel) but the reason why the Kanji Saba was used was because it sounds like Server, and actually originates from 自宅サーバー which means Home Server.
