It is possible to add flash documents to your website. If you already have the flash file (a .swf file) you can add it using the script box feature and a little code.
To put the flash on your page, upload the flash file (.swf) to your library and copy the URL to the file (this is the path to your file, you can find it at the bottom of your library screen when you click on your uploaded file).
Add a Html/Script Feature to the page you want to display the flash and inside include the following code:
Change the path to your flash file. The numbers, 600 and 310 refer to the width and height of your flash document.
This is definitely a little tricky at the moment. We plan to add a Flash feature so you can just add a Flash Feature to your page and select your document. So you can either try the technique above or wait for that.
All this assumes you already have your flash file. There are lots of sites out there which can help you build flash text messages and banners to get your .swf file.