Force Companion to Attack

You can set the target of your companions very easily by calling the "OnSlaveMasterNewTarget" dispatcher.

The dispatcher for OnSlaveMasterNewTarget

If you want to do it in a Skill Module, you could do it with a skill that has "requiresTarget" enabled, this means you have to select a Target to execute the skill, then the "Target" variable inside the skillmodule can be used to set it as the new target for all your companions.

The Skill Module for OnSlaveNewTarget