BlockReference ref_BLOCK_IN = new BlockReference(pt, btrId_in_EQ); // новый экземпляр блока "IN"
// получаем ссылку на выбранный блок OUT
BlockReference Ref_Selected_BLOCK_OUT =
trAdding.GetObject(entResult.ObjectId, OpenMode.ForWrite) as BlockReference;
// открываем на запись выбранный блок OUT
BlockTableRecord btRecord_BLOCK_OUT =
trAdding.GetObject(Ref_Selected_BLOCK_OUT.BlockTableRecord,OpenMode.ForWrite) as BlockTableRecord;
btRecord_BLOCK_OUT.AppendEntity(ref_BLOCK_IN);
trAdding.AddNewlyCreatedDBObject(ref_BLOCK_IN, true);
Ref_Selected_BLOCK_OUT.RecordGraphicsModified(true);
trAdding.Commit();