Have question about Katakana in Japanese!!!!!!?

Question:

Do I have to add an extra syllable when I type ā, ē, ī, ō, ū in Katakana like when I type in Hiragana?
For example, I have to add u after ō so I type ō as ou.

Answer:

Romaji is the only thing that uses ī, ō, etc.

When typing in katakana, long vowels are indicated with a ー mark:
ケーキ (cake)
タクシー (taxi)
テーブル (table)
etc.

When typing on your keyboard, remember that F6 will convert everything to hiragana (before you hit enter), F7 to katakana, etc. Hitting the space bar will show you a list of kanji options/etc.