The Kings aren't really in a position to trade away any of their defensemen. Kale Clague could be on the table since Hults is now an option for the Kings, but there's not as much of a market for LHD.
Roy was perhaps the best defenseman for LA this year, as
acknowledged by the Kings coaching staff. Walker is a different kind of defenseman than Roy, but his impressive skating ability and quick decision making are something that the Kings are sorely missing. If Roy is a more stable stay-at-home guy, Walker is a quality mobile puck mover, something that the Kings need with both Muzzin and Martinez departing over the last 18 months. I think both are exceptional 3RD options, but perhaps average 2RD options. On a contender, if either is a 2RD, your team might have defensive depth issues.
I'm not saying that the Kings wouldn't move either one, but it would need to be for something that makes the team better in the short-to-medium term, as they don't really have a replacement for either. They're not going to do a swap for Johnsson and Dermott or a draft pick, as that doesn't do anything to make the team better. I could see them doing a swap of Roy for Kapanen, which I acknowledge would likely be a bit of an overpayment in terms of value, but it would be made up for in the form of $2.5M in cap savings on Toronto's side. I don't think the Kings would add any picks or other pieces to offset.
So yes, Toronto could likely trade for either one of those guys, but they're not going to like what it costs.