But.....if you put the movies on the Time Capsule.....your "original" copies will be on the Time Capsule.
Are you remembering that you have no backups when, not if, the Time Capsule drive has a problem?
Suggest that you add a USB or FireWire drive and connect it to the MacBook. Then, Time Machine will back up both your MacBook and the hard drive.
So, you have "original" copies of your valuable data on one drive and backups on another. That is a minimum back up plan.
You can use Time Capsule for general storage as well as Time Machine backups. It is not a good idea though. Time Machine will eventually occupy all its free space. Once that occurs, copying additional non-backup files becomes impossible without erasing something else first.
"Half" implies you want to partition the TC's backup disk, which cannot be done.
This may work or it may not. Anything other than the TC's internal disk or a directly attached HD is not a supported Time Machine configuration:
Setting up Time Machine is as easy as connecting an external drive to your Mac via Thunderbolt, FireWire or USB. You can also use a secondary internal drive if your desktop Mac has one (that is, a drive that you don't start up from).
... Time Machine can’t backup to an external drive that's connected to an AirPort Extreme, Time Capsule, or a drive formatted for Microsoft Windows ...
Another alternative to Bob Timmons' suggestion is to add an external HDD to the TC for this purpose (although this would be slower than a HDD tied to your MacBook)
I generally do not recommend this type of setup because you still have a backup issue with the drive connected to the Time Capsule
The "original" data on the drive attached to the Time Capsule cannot be backed up by using Time Machine. So, you still have to use yet another backup solution like Carbon Copy Cloner or SuperDuper to get the movies backed up.
Another potential issue is that a hard drive connected to the USB port on the Time Capsule will run at about a third to half of normal USB speeds.
True, but the OP just wants a place to put his iTunes content, which is stored in the iTunes store anyway. TM doesn't need to back up to the drive.
Although, I would still recommend getting something like CrashPlan as then the data is stored off site, and using Dropbox for vital files. iCloud is also a good option.