It is possible to use your ASP.NET web parts in your Windows SharePoint Services (WSS) application simply by exporting them to create .webpart files.
.Webpart files are simply XML files that contains property values, state data, assembly or source file details about your Web Part (WP).
In order to be able to export your WP, you need to make some changes to your WP code and the configurationsettings.
Step by Step:
1) In your WP code, set the ExportMode property to allow properties to be exported, like this:
this.ExportMode = WebPartExportMode.All;
Locate and modify your system.web section in your web.config configuration file, like this:
Choose export from the Web Part menu to create a .webpart file which you now can import into your WSS WP Page :)