For the most part, NFL teams have done the bulk of their roster additions by this part of the offseason. Free agency and the NFL Draft were the two biggest opportunities to add pieces, but teams can and will look for ways to add to their roster between now and the start of the season.
Last year, the Steelers added ILB Kwon Alexander at the end of July and he ended up being a significant contributor until he suffered a season-ending injury in the middle of the season. Back in 2017, the Steelers traded for TE Vance McDonald toward the end of training camp.