If you have Unallocated Space in one of your Volumes, here is how to add it to an existing Volume, without loosing any data or needing to format the drives. 

Step 1: Open "Server Manager" go to "File and Storage Services" Then Click on "Storage Pools".

Step 2: You can check how much of your Drives are Allocated by Checking the "Capacity | Free Space | Percent Allocated" Section. Below I have "Storage Pool 2" which I will be using as my example, with drives totaling the amount of 164 TB with a 30 TB 2 Way Mirrored Volume. (60 TB Taken) 

Step 1: In the left hand corner click on the "Volumes" option and all of your Volumes will be displayed. 

Step 2: You can see, as I spoke of above, the "G" Volume is 30 TB Mirrored.

(Errors you might get) 

If you try to Extend the Volume of the "G" This box will pop up. You will notice that the Volumes "Current Size" and "Maximum Size" are the same. This is because I created the Volume this way and cannot exceed the limit of 30 TB.   

If you do attempt to make the Volume larger, as I am showing below, 

You will get this Pop Up stating "The new size must be less than or equal to the maximum size." 

Step 1: To make the Maximum size of the Volume larger, return to "Storage Pools" in the left hand menu.

Step 2: Right Click on the Virtual Disk the Volume you want to increase was created on (In my case it was titled "Virtual Disk 2") and select the option to "Extend Virtual Drive".  

Step 3: This box will pop up stating what the new Maximum Size may be and you may now reconfigure it to what size you need.

Figure 1: You will see the new Capacity and Allocated change to what you set them at. 

Figure 2: I decided to use the entire Storage Pool space so we only have 386 GB left of Free Space. 

Step 1: Return to the "Volumes" section in the left hand menu. 

Step 2: Right Click on the Volume you wanted to increase to begin with and you will see the new Maximum Size (Before it was 30 TB now it is 81.7 TB). You may now increase the Volume Size to what you see fit. 

Figure 1: You will now see your Original Volume, but with a larger Capacity.