As long as you have a desire to something else, and that something else has either short or flexible hours which you dictate (i.e. are self-employed / freelancer), you can do both... For example, you can be both a daytrader and a: a) prostitute b) journalist c) psychotherapist d) writer e) mercenary etc etc The list goes on and on and is only limited by your imagination... there is most certainly career life outside of daytrading... just pick your days and pursue the other career as a freelancer...
i'm not sure you can be a mercenary. hiding in a foxhole under enemy artillery fire doesn't seem compatible with daytrading. be a ninja instead, stay in the city.
If you wanted to trade carriers, do you need to find someone to actually take your job? What if you didn't want to be what that person was, do you still have to trade? Is there a web site that lists jobs for trade?