Hello ugo,
If the alignment of the project has been performed while "Keep key points" option is turned on in the Advanced tab of PhotoScan Preferences window, it would be possible to add new photos to alignment. Add new photos to the chunk, run Align Photos operation with "Reset current alignment" option unchecked.
Note that it wouldn't be possible, if the key points were not stored in the project, then it would be necessary to re-align the complete image set from scratch.