The proper way would be to install 98 first and then XP; however it is possible to load 98 on an existing XP install if the c drive is formatted fat or fat32, not ntfs. There are instructions at DougKnox.com.
Just do a search for installing 98 after XP.
Keep in mind also that there's usually no need to have Windows 98 to get older games to work under Windows XP. You simply need to do a bit of research and find out how others have been able to make the games work. For C&C: http://www.gidforums.com/t-4535.html