As Ken noted on Visual FoxPro: January 2006 - Letter from the Editor, the XSOURCE.Zip file for VFP 9 has its own license agreement, which includes the right to modify and redistribute.
This is great news - why? Because now you don't have to try and reinvent the wheel, you can simply enhance it without worrying about breaking it.
For those who like to play with the EULA - this is a Microsoft Permissive License that says "Microsoft grants you a non-exclusive, worldwide, royalty-free patent license under licensed patents to make, have made, use, practice, sell, and offer for sale, and/or otherwise dispose of the software or derivative works of the software." as well as "Microsoft grants you a non-exclusive, worldwide, royalty-free copyright license to reproduce the software, prepare derivative works of the software and distribute the software or any derivative works that you create."