What do you mean? As long as you're on an everything unlimited Simple Mobile plan, you'll have data, so you can use iMessages with that. Or did you mean something else
iMesssages uses data/wifi to message. As long as you have data/wifi access when you send the message to another i-user, it'll go through iMessages instead of regular SMS.
One point of clarification. And I only bring this up because I forced my daughter to do it. iOS and OSX users can turn off iMessage. Which means even though they have it, they will only get SMS and MMS from you.