The following query should perform the requested:
INSERT tblAssetUserRelations (Username, Userdomain, AssetID, Type, LastChanged, StartDate)
SELECT Username, Userdomain, AssetID, 1, GETDATE(), GETDATE() FROM tblAssets WHERE Username + Userdomain IN (SELECT Username + Userdomain FROM tblADusers)
Instead of using GETDATE(), you can set a custom date using this format: '2014-06-18 00:00:00.000'. The type of relation can be found in tsysAssetRelationTypes, 1 being 'owned by'.