It's now been confirmed by People magazine that Cobie is slated to appear as part of the "hidden-camera shenanigans involving NPH's celebrity pals". Here's the paragraph relating to it:
Harris' unsuspecting celebrity victims include The Voice's bromantic buddies Blake Shelton and Adam Levine, plus Modern Family's Jesse Tyler Ferguson, Unbreakable Kimmy Schmidt's Jane Krakowski and Harris' former How I Met Your Mother costar Cobie Smulders (who's directed to do some pretty uncomfortable stuff by her partner, Saturday Night Live star Taran Killam).
You can read the full article on the show and watch a first look video at the
source site.
When you watch the video it shows a clip of Cobie's part and it's actually both Taran and Neil directing her to do embarrasing things with the people she's around who don't know she's wearing an earpiece. It's very similar to Howie Mandel's recent hidden camera show Deal With It, and it's cringe-worthy!