The following method can be used to assign a large number value of int64 datatype to an Active Roles Virtual Attribute of Integer8 syntax named edsvaInteger8:
[int64]$largeNumber = 2286677030836249000
$objLargeInteger = New-Object -ComObject AelitaEDM.EDMLargeInteger
$objLargeInteger.SetString($largeNumber)
$intString = $objLargeInteger.GetString()
Set-QADUser "CN=John McTest,OU=One Account,DC=domain,DC=local" -ObjectAttributes @{edsvaInteger8=$intString}