Hi,
Right now there is no easy way to import excel to an existed worksheet. To do this, you can first import excel file to a new book or sheet by impExcel, and then copy and paste it to the existed sheet using some scripts.
To turn off the import message dump to the Results Log window, please uncheck the Results Log Output checkbox in the import X-Function dialog. If using LabTalk script to import, impExcel for example, you can use
impExcel -sl fname:="F:\OriginLab\Origin8\Samples\Graphing\Excel Data.xls" desig:=<Unchanged>;
to import excel without dumping results to Results Log.