I've had the same problem recently. I could solve like this:
1) in CRX create a node with property 'jcr:primaryType' = 'sling:Folder' (let's call it 'xy-lookup-list')
2) add a multi string property containing the key-value-pairs to the newly created node. The property name (let's call it 'xy-lookup-values') will be part of the loading path. A key-value pair looks like this 'keyXY=valueXY'.
3) specify the crx path as loading path in the dialog of the widget that shall use the lookup value. The loadpath is a 'crx' path like '/content/dam/.../xy-lookup-list/xy-lookup-values'. Make sure the last two elements of the path are the names of the node and the attribute.