Hello!

There indeed is an easy way of doing this. Since you already have a key for your Death Grip bind, all you need to do is check a checkbox for it in step 1 of the mapped key. You can see the location of the checkbox in this image just below the target and key combination selections on the right side panel. In this example "E" would correspond to my ingame Death Grip bind.

Depending on how you've set it up now, you might want to also make sure that the target of the mapped key only involves your DKs, as the Paladin does not have a deathgrip, so sending the command to it is somewhat pointless in most cases. You'd most likely simply want to create an action target group for the DKs, apply the characters to it and as in the example image, make the target of the mapped key "Group (all of) Death Knight".