depending on the current McAfee Agent Extension and Agent version, you might be able to accomplish this by setting the "Updates" configuration in the McAfee Agent policy.
Duplicate the current policy and name it to something that will identify it so you know it is directing the client to get software from a different branch, in your case evaluation.
Assign this policy to the target workstation. Create a client task to update the product.
Sometimes, you can create a new branch in your system tree and assign this policy there. And just drop workstations in there.
You must be running McAfee Agent 4.5 Patch 1. Within the McAfee Agent policy, set the Host IPS product to a different branch under Patches and Service Packs. Update the policy on the client, and then run an update task.
If you are not running McAfee Agent 4.5 Patch 1, please see: KB53025 - McAfee Patches cannot be deployed from either the Previous or Evaluation Branch