We do not have the ability to select a specific carrier, but there is a crude method of accomplishing this when it is required (very rare).
When placing the order in Magento, simply add an address that's guaranteed to be held for verification in Shipwire. My go-to is simply write in "HOLD ORDER" on the address lines and when the order shows up in Shipwire it will be held. At this point, you can change to the correct address and select the carrier.
I would not recommend doing this often as it is time consuming and the vast majority of the time unnecessary. The only time I tend to use this is when a customer cannot receive a specific carrier in their area.