I think you are going to need to pull that info from the tblADUsers. Link the username from tblAssets to the username on tblADUsers.
I have updated your code. see if this helps:;
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetUnique,
tsysOS.Image As icon,
tblAssets.Domain,
tsysOS.OSname,
tblAssets.Description,
tblAssets.Username,
tblAssets.Userdomain,
tblAssets.Memory,
Cast(Cast(Cast(tblDiskdrives.Size As bigint) / 1024 / 1024 / 1024 As
numeric) As nvarchar) As [Disk size],
tblAssets.NrProcessors As #CPU,
tblAssets.Processor,
tblAssetCustom.Model,
tblAssetCustom.Manufacturer,
tblAssetCustom.Serialnumber As Serial,
tblAssets.IPAddress As [IP Address],
tblAssetCustom.PurchaseDate,
tblAssetCustom.Warrantydate,
tblAssets.Lastseen,
tblAssets.Uptime,
tblAssets.SP,
tblBIOS.Version,
tblAssets.Mac,
tblAssetCustom.Department,
tblDiskdrives.Freespace,
tblDiskdrives.DriveType,
tblAssets.SiteID,
tblAssetCustom.Location,
tblAssetCustom.Branchoffice,
tblAssetCustom.Building,
tblADusers.Office,
tblADusers.Department As Department1,
tblADusers.Company As Company1,
tblADusers.Title
From tblAssets
Inner Join tblDiskdrives On tblAssets.AssetID = tblDiskdrives.AssetID
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysOS On tblAssets.OScode = tsysOS.OScode
Inner Join tblBIOS On tblAssets.AssetID = tblBIOS.AssetID
Inner Join tblADComputers On tblAssets.AssetID = tblADComputers.AssetID
Inner Join tblADusers On tblAssets.Username = tblADusers.Username
Where tblDiskdrives.Caption = 'c:' And tblAssetCustom.State = 1