Hi Octavia!! I'm Non, Japanese. I also agree with Kirsten. There is nothing wrong if you make sentences only in hiragana and katakana, because I think you aren't able to use kanji unless you have some knowledges of kanji. First of all, I suggest you should be used to using hiragana and katakana in writing. After that, you can change some words of hiragana for kanji. I reckon it's gonna be efficient, because that kanji could be related to what you want to express is easy for you to remember.
Sorry, but I'm not good at computer, so I have no idea about that app...And sorry for my bad English. I hope you will understand...