Essentially, not that I'm aware of. The REx, once it turns on, doesn't try to raise the SOC, only to maintain it. So, while it will come on automatically if your SOC reaches 6% (or higher, if you've coded it and remembered to turn it on), whenever it came on is the maximum it will try to bring the batteries. Note also that if you turn the car off, whatever that level is becomes the maximum SOC it will try to achieve.
While the i8 and some other vehicles are designed to be able to fully charge the battery from the engine, they all have MUCH larger engines that are capable of propelling the vehicle all on its own. The i3's engine is pretty small, and the gas tank is, too.
Your best bet is to code your vehicle (if required) to come on at a higher level (75% is the max) and remember to turn it on then. Then, if you stop later, your battery will still be fairly full, and if you buy some more fuel, you can keep it there until you can recharge it later.
Note, if your SOC is quite low when you stop and leave it, if it's really cold out, you could damage the batteries if it sits for a long time and everything gets frigid.
