Lyons is a 6-5, 295-pound rookie who originally signed with the Tampa Bay Bucs after this past draft.
A Division II All-America from Tusculum College -- the school that gave the Steelers second- rounder Ricardo Colclough in 2004 -- Lyons set single-season school records for tackles (89) and tackles-for-loss (19) by a lineman last year.
At his pro day, Lyons ran a 4.86 40 with a 30.5 vertical jump and a 10-foot broad jump.
This preseason, Lyons played in 101 snaps for Tampa Bay and had one sack, four QB hits, four QB hurries and seven tackles.
Todman is the fourth-year running back from Carolina who scorched the Steelers with a 49-yard touchdown run in the preseason finale. Todman rushed for 72 yards on five carries in that game and 134 yards on 17 carries the entire preseason.
A sixth-round pick by San Diego in 2011 out of UConn, Todman spent the last three years with Jacksonville and rushed for 450 yards (4.1. avg.) on 111 carries. He also caught 40 passes (7.9avg.) and returned 69 kickoffs (25.9 avg.).
The Steelers also signed the following 10 players to their practice squad:
RB Dominique Brown, SS Jordan Dangerfield, OL Reese Dismukes and Antonio Johnson, LBs L.J. Fort and Shayon Green, DE Ethan Hemer, WRs Shakim Phillips and Jarrod West, and TE Harold Spears.
The Steelers presumably will add someone to replace Maurkice Pouncey on the roster, since Pouncey can be placed on Injured Reserve with a designation to return in the second half of the season. The Steelers are likely waiting until the second week to make the move in order to avoid guaranteeing a full season's contract to a vested veteran.