Is there a way to add and align images to the original sparse point cloud?
Yes I think it can be done with a little trick:
Load the images you shot afterwards, for example from a detail, in a new chunk and add also some from your original shooting which overlap with them. Process this new chunk as normal. Then you have some common photos in both chunks and it should be posible to combine these chunk.
Never done this but it should work.
If you have the Pro version of PhotoScan you can also merge these chunks by markers.
Regards
Karsten