This is the only way to expand an existing striped metadevice.
If you use DiskSuite Tool to drag multiple slices into an existing striped metadevice, you are given the optional of making the slices into a concatenation or a stripe. If you use the metattach(1M) command to add multiple slices to an existing striped metadevice, they must be added as a stripe.
At the stripe level, using either the Stripe Information window in DiskSuite Tool, or the -i option to the metattach(1M) command. Each stripe within the concatenated stripe can have its own interlace value. When you create a concatenated stripe from scratch, if you do not specify an interlace value for a particular stripe, it inherits the interlace value from the stripe before it.