I enquired about this when I bought my 6ft hutch over run as I wasnt sure I wanted the run. They do sell extra long legs, but to be honest you need 6 of them not 4. I did get the hutch over run but being so long mine has a slight bow in the middle, which is corrected by adding an extra leg.
I really dont like low to the ground hutches, makes it too easy for foxes to scratch and chew at.