Creating Excel files in SSIS based with a predefined format
If your SSIS package runs with the appropriate file system permissions, you can create a template Excel file with the desired formatting and copy that file to a new one (using a file system or script task) where the data will be exported. If the sheet will contain a variable number of cells that must be formatted, you can use conditional formatting in the template and have Excel apply the format based on actual column values or a formatting flag, say in a hidden column, which could be populated from the data.

See this article for an example of using a template. This article may also be helpful; it mentions an ingenious way to update specific cells.