In order to convert a legacy application you will need to login to either your sequencing server or your publishing server.
Launch PowerShell (x86) and run as an administrator.
You will need to load in the App-V module beforehand, the command you need is: import-module AppVPkgConverter
Locate your Legacy application and make a note of the UNC path, you will also need to take a note of the UNC path for the destination folder.
The command you will need to run is below.
ConvertFrom-AppvLegacyPackage –SourcePath \\YourUNCPath -DestinationPath \\YourUNCPath -OSDsToIncludeInPackage X.osd,Y.osd