I use the Anthy IME with ibus. I know that I can switch a word to Katakana with the F7 key but when I do this, any long vowels are kept as two regular syllable katakanas. That is, わあ (waa -- this is not a real word as far as I know) would be converted to ワア. I need to replace the ア by the lengthening mark which looks similar to a hyphen. How to do this?
Use the hyphen for the prolonged sound mark. In your case you would type "wa-". Even if you don't use the Katakana mode, on pressing space the hyphen is replaced by the "ー".