No, you can only work with the Excel extension within Excel. However, you can import other types of files into Excel, work on it while it is inside of Excel, then export it back out, and replace it in the original program application.
To do this you need to use the Excel Import function. On the Data menu hover on Import External Data. Select the option on the submenu which most closely fits your need Then, follow the steps in the import/export Wizard. Once inside of Excel, do whatever you need to do with the data, then when you are finished, use the Export Wizard to export it out of Excel and place it back in the application it came from.
Now, you can use the Open dialog box to do some very simple importing and exporting of external data types. Open it up and then click around and check out the options.
The Import External Data option is very detailed in what it can do, and has many different options. I strongly suggest you utilize the Office Online Help option. Once at Office Online you can simply put in help queries and locate a load of topics related to importing/exporting and working with external data. There are even lessons you can do for free!
You can also use the quick search function at the upper right corner of Excel. Type in your query, and all the available topics, on your system, on your network if you have it configured this way, and at Office Online, will appear in the task pane at the right of the Excel window. Just scroll down the list and click on the topics which resemble the task you wish to complete.
Good luck and much success with your work. Please have a nice day.
*edit* I highly recommend a book from Microsoft Press called Excel Inside Out. If you go to chapter 29 it will provide detailed information on data base and other external data you can work with in Excel. I find it extremely valuable, not only for this section, but for all the other major topics it covers in easy to read/understand format and with steps on how to complete the tasks you wish. It is well worth its $45.00 price tag. It also has a CD with an electronic copy of the book, and many other valuable items, such as Macros, Scripts, utilities and tools. You can find this book and other Office Inside Out book at any local bookstore, or by ordering directly from Microsoft Press.