I don't believe such a rule exists. I remember Texas Tech adding a kicker to the roster mid-way through the year from one of those student-kicks-a-FG-for-a-prize things a few years back.
And I agree that surely there's got to be a few people on campus, football players or otherwise, who could walk on and add a few minutes of depth apiece.
So we're taking only 8 to Hawai'i...what happens if someone gets hurt and then 3 guys foul out? Do we play with 4, or do we forfeit? Either way, we need a few more bodies on the bench.