Adding external photos to iPhone library in correct order
Non tested manual method


create a new folder ALL_PHOTOS on your PC/Mac
copy all your iphone photos directly to ALL_PHOTOS
copy all your camera photos directly to ALL_PHOTOS (no subfolders)
go to iTunes, select your iDevice, go to Photos section, check 'Sync Photos' checkbox and choose the ALL_PHOTOS folder as your 'copy photos from' folder.  
Sync


After sync done, all your photos should appear as a new album in Photos app. Then you can safely delete photos from Camera album on your iPhone. Repeat this procedure once in a while.

Downsides:
1. 100% manual, slow and boring procedure
2. an annoying problem with duplicate filenames can arise