You can, yeah. It's a bit of an odd process to get your head around though. If you go to the analog data tag, you can put the value you desire in the "Force Mask" column. The strange part to grasp is that you are forcing it and the output signal is responding, but it doesn't show you that *anywhere* in the tagname or code though. So if you have an output that activates an alarm and you force it, you won't get the alarm and anywhere in the code that output value is shown, it won't be your forced value.
You can imagine how frustrating that can be! Literally following the manual step for step and not seeing the output change in Logix. But if you clamp a meter to your output you'll see it change.