Careful. If you delete the NTFS partition, it should now show up as free space. You can then grab the lower right corner of your TM partition and "drag" it to fill the drive and expanding it to fill the drive shouldn't lose any data. However, if the partitions aren't adjacent to one another, you probably can't combine them without wiping the drive (and doing without your TM backups until you let TM back everything up all over again).
I have had to wipe my TM backups several times. It's no big deal as long as your main HDD doesn't die before TM has time to back it all up again. Apple support actually convinced me to delete it the first time when my destination volume got corrupted due to a bug that dealt with the Mac going to sleep during a wifi backup session. The bug has since been patched.