Yes, it is possible, but there are a couple of thinigs you should keep in mind.
If your site is on a web hosts Linux/UNIX server that supports FrontPage Server Extensions and you originally uploaded the site using FrontPage, if you upload the modified pages using anything other than FrontPage you'll alter the folder/file permissions on the web server and effectively kill the site for future editing/uploading in FrontPage.
If you had the option to use FrontPage Server Extensions disabled during your design stage and you don't use the extensions and don't plan to in the future then you can use any FTP program you like.
Some web hosts will reset the web root folder if you use FTP then decide to go back to FrontPage, while others will either refuse to do so or will charge you for the trouble.
FrontPage 2003 features a code optimise facility which cleans and orders everything very nicely prior to publishing. You could also edit the files locally from their FrontPage root folder on your hard drive with another application, then upload them to your web space via FrontPage, to maintain tyhose extensions for future use. With a bt of lateral thinking you can get around the issue, but it's only fair to make you aware of it from the outset.
The short answer is that you can use any FTP program or web editor that features built in uploading facilities but if you plan on using FrontPage Server EXtensions at any stage for feedback forms and other nice things, you are best sticking with FrontPage for both editing and uploading/publishing.