Hey,
You can absolutely use the Alpha channel alongside the RGB for packed textures. Best method is to pack them in photoshop or any good image program, and ensure you export as a 32bit targa. In photoshop just add new channel and it will add a alpha channel to your RGB document.
In Unreal, have ‘use SRBG’ disabled in the texture editor. Also double check at the top it says has alpha channel : true. You can preview each channel with the channel dropdown on the top left of the texture editor viewport.