I assume you are adding more disks, right?  In that case, at least with other models of DD, you would add license first, add disks and then you would extended it with storage add command.  However, this would always create second dg.  At the end you extend file systems with filesys expand. If for whatever reason you wish to have 1dg (no reason really), then you would need to reinstall DDOS. On DD site you can search for Storage Expansion which will give you docs with more details.

Yes , I'm going to add more disks. But ,if create a second dg , it will cost 2 disk's capacity. And the customer really cares about that.

With old system such like 4.x ,it is possiable to destroy the fs and dg,than create a new dg with all the disk.That will give more useable capacity. With DDOS 5.x , I'm not sure how to do.

In 5 you can also escape to shell stop fs, destroy the things and create from scratch.  As you may be aware, such thing is usually meant to be executed by DD personnel so you would need to have ticket with those folks.  In both cases (DDOS reinstall or this kind of extension with fs and dg being destroyed, you can say goodbye to your data unless you have copy somewhere else and can pull it back after extension is done).

As Mr Hrvoje Crvelin told you already ,

You have to destroy the filesystem (loose all backed up data) and build it from scratch with the new disks, I had the same disscusion with a customer a couple of weeks ago since they also cared about the usable space! (Customers tend to this quite often )

So copy / replicate the backup data from the DataDomain  and destroy and re-build Filesystem.

OK.The data is not the problem. So ,how to do it in detail? Just use 'filesys destroy' and 'filesys create' ?

Destroy filesystem:

#filesys destroy

or

#filesys destroy and-zero

Add storage enclosures

#storage add

Create filesystem

#filesys create

Enable filesystem

#filesys enable

Verify:

#filesys show space