Yes, modifications made to an Excel table joined with a feature class or shapefile in ArcMap update dynamically. However, prior to making modifications, ArcMap must be closed. This is by design, as permissions to the Excel table are set to read-only when the table is in use by ArcMap. Once modifications to the Excel file are completed, ArcMap can be reopened and the modified records are updated in the joined feature class or shapefile. For more information, please refer to the following web help topic: ArcGIS Help: Understanding how to use Microsoft Excel files in ArcGIS.
The use of macros and other formatting methods may cause unexpected behavior during the join process. Prior to making the modifications, please refer to ArcGIS Help: Formatting a table in Microsoft Excel for use in ArcGIS.