This switch does support a wired SFP+ stacking cable -
1M SFP+ Interconnect Cable
SFP+ interconnect cable supporting 1Gbps and 2.5Gbps and 10Gbps, SFP/SFP+ (1 Meter). For use with the NetVanta 1550 series.
But currently, there are not any SFP+ copper modules that are tested and supported.
This may be changing soon and the switch does not stop a third-party SFP+ copper module from operating, but not all modules may be compatible. Check with your ADTRAN Sales Engineer to request an SFP+ copper module to be added for this use.