Here is a pinout of the smart ports:
They use UART over RS-485, which is a half-duplex serial link with two data pins (A and B). The power pins (+ and -) are connected directly to the V5 battery lines, so as James said they are much higher voltage than you probably want (somewhere in the range of 13 V). Also, they cannot be switched on or off. To reasonably control one or more LEDs from a smart port, you’d need some custom electronics to interpret commands sent over the RS-485 link.
The only way to directly connect an LED to the V5 and be able to control it is via any of the 8 “legacy” ports (A through H). I also cannot comment on the exact legalities of using such a custom device in competition (I’m more familiar with the relevant rules for VEX U), but if you’re not sure after reviewing the game manual, I’d encourage you to open a Q&A question.