No, there's no way to change chat mode to shout or yell because it's annoying lol. When you use shout, everyone in the entire zone can hear you, when you use yell, everyone even in surrounding zones can hear you (this might not be 100% correct, but close enough for my point). They make it so you have to manually make it shout/yell so that everyone doesn't accidentally type stuff in these modes, and to dissuade people from over using them. Sure, the occasional shout conversation can be interesting, but for the most part, if people wanted to hear your conversations, they'd be in your Linkshell.