There's nothing stopping the Pats from signing him now to a futures contract. He's not eligible to play this year in the playoffs, based on an NFL rule enjoining CFL players from playing in the NFL in the same season.
Adam Schefter says that Indy and Philly also worked out Armstead, and thinks he will be the equivalent of a "high draft pick":
Armond Armstead, formerly of Toronto Argonauts, on NFL teams' radar -- sources - ESPN
If that assessment is correct, Armstead may be viewed as a potential starting DE/DT similar to how Jonathan Fanene was viewed a year ago, but 7 years younger, and with much higher upside. Add a talent like that - again, assuming Schefter's assessment is correct - to our current DL, and the Pats have suddenly significantly upgraded their DL.
Remember also, the Pats won the highly contested Jeff Demps and Brandon Bolden derbies, so BB clearly has some ability to persuade sought after UDFAs that the Pats offer the best opportunities for personal and team success. Sith mind tricks, no doubt. I think he could reasonably convince Armstead that he has a shot of playing a significant role on the team, and earning considerable playing time. The rest is a matter of money.