The show was on-air before Richard's story line so there is a chance he already knew of the stalkers.
But, as you can hear in this sound-byte, SubZero is introduced in detail so Richards could have simply heard/seen it then.
Related:
In the book, Richards willingly goes to the network which is where he learns about everything (including the stalkers) beforehand.
After rigorous physical and mental testing, Richards is selected to
appear on The Running Man, the Games Network's most popular,
lucrative, and dangerous program. He meets with Dan Killian, the
executive producer of the program, who describes the challenges he
will face once the game begins.