If you want your game to always load every sound, that defeats the purpose of event-based packaging. If that's what you want, you're better off with the old Sound Bank system. You just create one big Sound Bank in Wwise, load the soundbank in your game at init, and voila! You can call any sound any time with a string.
Event-based packaging works on the idea of Unreal only loading exactly what sounds it needs, to save memory. So if an actor/component/etc.. has an AkAudioEvent loaded on it, it will load when the level loads, or when the object gets spawned, etc..
But maybe you like event based packaging most of the time, but you'd like to to things the old-fashioned way in some situations.
Here's some ideas:
use LoadObject to get the AkAudioEvent you need, when you need it. Then it will be loaded, and PostEvent will work.
UAkAudioEvent* Event = LoadObject(null, "Game/WwiseAudio/path/to/objectName.objectName")
You could hold events like these in a data structure, so you don't have to Load them every time. You could even implement some "garbage collection" to Unload events that haven't
been used in a while.
Another idea (less certain of this one, but it works in theory): Would be to simply use an AkSoundBank for some of your sounds. Create the banks in Wwise, then next time you sync assets in Unreal, you'll get the bank you created as an AkAudioBank, and you can load it in code at Initialization, or have it referenced by a blueprint that's instantiated in your level, and that should auto-load it.