If third party control is in DLL form and can be created using the standard API CreateWindowEx, then yes.
You need to put the third party control DLL in the same folder as the Visual Designer. Make sure when you generate a project, it uses the property to copy all DLL's, not just EZGUI's runtimes to your projects source code folder.
You will then need an EZC file to define the third party control, so the designer will recognize it and load it. EZC files are also put into the same folder as the designer.
For more info about how to create an EZC file for a third party control see:http://chrisboss.hypermart.net/ubb/Forum28/HTML/000020.htmlhttp://chrisboss.hypermart.net/ubb/Forum30/HTML/000001.html
These posts are on my old support forums (which are closed to new posts, but there for reference).