I agree with Sephe about not editing the data outside of Arc. A join within ArcMap is a temporary link between the datasets. Generally, I prefer to join the data (perhaps to see what matches or doesn't match) then if I like the results I will export it so that it is a separate static layer. Although possible to edit the layer, I don't think it would not be possible to edit the excel data since that is data being joined from another dataset that ArcMap knows very little about. Exporting the data will create a layer with Object IDs and all the fields you have turned on when exporting the join, after exporting the data is in an ArcGIS format. Performance should be better on the exported layer too. This will also remove data locks on your Excel spreadsheet should you need to edit it. However, remember that the export is a static layer and will not reflect changes to your Excel data after exporting. If you need a dynamic layer that joins your excel data and GIS data, then I would suggest looking into SDE, where you can create spatial views that are basically dynamic joins where the data is managed by a RDBMS like SQL Server.
