Using the Camera Connection Kit, you can connect any of our USB Audio devices to your iPhone. It will take the place of your microphone, you'll need to grant permission when prompted.
Read this article to learn more on where to find this setting in iOS (article written for iPad, still applies to iPhone)
While PreSonus does not offer any audio recording applications for the iPhone / iPod Touch series there is an assortment of recording and audio applications available on the Apple app store that you can use with your iPhone.