It's certainly possible to break a drive into 2 partitions (e.g. having a WBFS part and a FAT32 part), and use it for both purposes (WBFS for gaming, and FAT32 for homebrew). EASEUS partition manager is a freeware application that quite easily allows you to make that repartition.
However...considering that WBFS isn't supported by it (or by just about any other partitioning program), the chances of keeping the data on your drive is low, even if there's still plenty of free space. In fact, the chance is probably even zero.
Of course, you could restore your wii games to your hard disk, use EASEUS to break it into two FAT partitions, use WBFSmanager (I think) to format the first part into WBFS, install all games to that partition and copy all homebrew to the remaining FAT32 partition...
...but seriously: if you're making changes of this size, you should at least consider just using a single FAT32 partition. It's not more work than breaking it into 2 partitions and the result is at least more elegant.