Spillman is expected to take the place of special teams player Curtis Taylor, who was injured Sunday night and is expected to miss five weeks with a torn right quadriceps muscle, according to CSN Bay Area.
Fullback Jehuu Caulcrick was also released from the 49ers' practice squad. If Brock clears waivers, he's expected to be signed to the practice squad. Brock had been the team's nickel back but lost that job when cornerback Will James returned from an ankle injury.
It's uncertain how much Spillman will help. He totaled six special teams tackles in five games, but the Chargers' special teams played horribly Sunday in a 35-27 loss to the Oakland Raiders.