Keywording in the “File Browser” side of the app is on the list for a future release. However, I’d like to just have a single keyword feature for both Photos and File Browser but of course that depends on Apple.
BTW: While you may only want to use Photos for your iPhone images, you could consider some of Photos’ advantages for your other images, especially paired with RAW Power. The biggest of course is iCloud Photos support. I don’t have immediately plans to create albums and stuff like that on the File Browser side – while those are already provided in the Photo library.