Microsoft Virtual Machine Converter can be used to convert non-MS virtual machines to Hyper-V using a wizard or CMDLets and non-VHD hard disks to VHD or VHDX using CMDLets.
A walkthrough is posted here: https://technet.microsoft.com/en-us/library/dn874008.aspx
If you wish to convert a VM for usage with Windows Desktop Hyper-V, you will need to use the CMDLets to convert just the disk file.
CMDLet reference is here: http://download.microsoft.com/download/9/1/E/91E9F42C-3F1F-4AD9-92B7-8DD65DA3B0C2/MVMC_cmdlets.doc