It appears that the character is limited to Katakana. Is the character used in writing Hiragana and Kanji?
You're right that it is normally used in katakana, but it can sometimes be used in hiragana words, as a point of emphasis, to lengthen a sound more than it normally would be:
You can add it to い adjectives. Otherwise, the word in question is generally written out in hiragana rather than in kanji, but it is apparently alright to add it directly to a kanji for emphasis or silliness: