1

Add Armature

Select the object with the shape key animation and press shift + A
2

Add Single Bone

Go to, armature and add a single bone This drops the bone in hierarchy, but it’s not connected to the object
3

Select Both Objects

Select both, the object and the armature (bone), first select the armature, then the object
4

Parent Objects

Then press command + P to parent
5

Choose Automatic Weights

Select ‘with automatic weights’
6

Export Settings

For export select the armature in the hierarchy and choose ‘Selected Objects’ under Include ‘Limit to’ in the export settings.