02 February 2018
Minor correction here, it is possible to add disks to an array, change raid level, or increase the size of a logical drive in an array under certain circumstances (the disk space must be contiguous, if many logical drives were created and some deleted, the free space will be fragmented and you can't expand into it all). I don't know the order of the logical drives in your Pegasus, but if you only had 2 LDs (Logical Drives) and you delete the last LD you should have all the remaining space available to migrate into. You can't make a LD smaller or remove disks from an array.
This functionality is accessed through the 'migrate' function. You can find this in the background tasks window, which is on top to the far right of the Promise Utility.
Be advised that migration can be slow and that it cannot be stopped or paused once started.
Also, since the Pegasus will go to sleep when the MAC goes to sleep, you probably want to disable that while the migration is in progress. I'd let the screen saver blank the screen, but the sleep needs to be turned off on the macOS until the migration is complete.
The migrating LD can be used while the migration is in progress, but it will impact performance. This may or may not be a problem depending on what tasks your Pegasus is being put to.