To add a new clip to your current project
Step 1 - Select where you'd like to add your clip
Tap on the timeline
Adjust the timeline to where you'd like to insert a clip
Select Split and Apply
Step 2 - Select your clip
Tap the icon between your two split clips
Tap Add
Select your clip from the Photos app
Note: Captions will not be generated for the added clip, but you can add captions manually.