If you run UnrealPak.exe without any parameters, it shows this usage:
Usage:
UnrealPak <PakFilename> -Test
UnrealPak <PakFilename> -List
UnrealPak <PakFilename> <GameUProjectName> <GameFolderName> -ExportDependencies=<OutputFileBase> -NoAssetRegistryCache -ForceDependsGathering
UnrealPak <PakFilename> -Extract <ExtractDir>
UnrealPak <PakFilename> -Create=<ResponseFile> [Options]
UnrealPak <PakFilename> -Dest=<MountPoint>
UnrealPak GenerateKeys=<KeyFilename>
UnrealPak GeneratePrimeTable=<KeyFilename> [-TableMax=<N>]
UnrealPak <PakFilename1> <PakFilename2> -diff
UnrealPak -TestEncryption
Options:
-blocksize=<BlockSize>
-bitwindow=<BitWindow>
-compress
-encrypt
-order=<OrderingFile>
-diff (requires 2 filenames first)
-enginedir (specify engine dir for when using ini encryption configs)
-projectdir (specify project dir for when using ini encryption configs)
-encryptionini (specify ini base name to gather encryption settings from)
-encryptindex (encrypt the pak file index, making it unusable in unrealpak without supplying the key)
In your case, you would want to run:
> Engine\Binaries\Win64\UnrealPak.exe X:\dest\somepak.pak -Create=X:\source\ -compress