New and existing learners cannot be added to an existing learning block.
Once the learning path, or block, has been created and is currently active, you cannot add a new learner to it.
You can edit the timeframe of the active learning path.
If the learning path has already started, then you will need to schedule the individual(s) into their own learning paths.
If the learning path hasn't started yet, then you will need to delete the existing learning block (to avoid any duplication) and schedule a new learning path for the new individuals.