You will need to create a new lun and use the new lun as an extent for your old lun. My recommendation would be to create a larger new lun and migrate your VM (s) off your current one onto the new one, then delete your old lun.

I believe in the next version of ESX (vSphere) there is some new functionality for extending a vmfs volume without using extents  "Online VMFS expansion - handy, so if you can extend a LUN from your array you can grow the VMFS into it online without downtime, up until now the only alternative was downtime or storage vMotion to a brand-new LUN, or to use extents which are not as safe"

http://vinf.net/2009/02/26/hands-on-lab-01-vsphere-features-overview/

Message was edited by: java_cat33 - added vSphere info

use add an extend to your existing datastore:

Re: Add Extent - Expand VMFS volume....

vmkfstools -Z vmhba1:0:12:1 vmhba1:0:11:1

or via GUI:

Adding Extents to Datastores

You can expand a VMFS datastore by attaching a hard disk partition as an extent.

To add one or more extents to a datastore’s spanned volumes

1.  Click the arrow next to the Inventory button in the navigation bar and choose Hosts and Clusters.

2.  Select the host and click the Configuration tab.

3.  Click Storage.

4.  In the Storage list, right-click a datastore and choose Properties.

5.  Click Add Extent.

6.  On the Extent Device page, select the disk to add as the new extent and click Next. 

7.  Review the current layout of the disk you are using for the extent to make sure the disk doesn’t contain important information.

If a disk or partition you add was formatted previously, it will be reformatted and lose the file systems and any data it contained.

8.  Click Next. 

9.  On the Extent Size page, set the capacity for the extent. 

By default, the entire free space available on the storage device is offered to you.

10.  Click Next. 

11.  Review the proposed extent layout and the new configuration of your datastore and click Finish.

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