To add photos to existing albums you can't do from the native FB app like you can on iOS. On this app if you create a post first and hope that there'll be a drop down box to choose album, you'll be disappointed.
To add photos to an existing album you'll have to navigate to the Posts & Stories icon at the bottom then select Photo. From there choose album to upload then select Add Photos. If this was smooth and streamlined as the iOS app I would have given 5 stars.