I'm not sure of anyway to do it directly from OVFTool, but if you use a PowerShell script to call your existing OVFTool command, and add this after, you could set the description after the import that way:
Get-CIVAppTemplate -Name "vapp_name" | Set-CIVAppTemplate -Description "description"
You might also consider just using the Import-CIVappTemplate cmdlet instead of OVFTool and set the description at the same time using the -Description parameter. (that is, if PowerCLI is palatable to you and a viable alternative)