Can I create a fusion drive with an HDD that already has a boot camp partition?

From my hands-on experience, I'd have to say, "negative".

I've created a couple of fusion drives on a 2008 Mac Pro from devices with preexisting partitions. Building the fusion in Core Storage destroyed all partitions on the media.

With trepidation, and after a full backup, I specified two arbitrary slices from two already-partitioned hard drives:

diskutil cs create <myTestVolumeName> disk0s2 disk1s4

Because I picked two partitions I could sacrifice, the HD partition (20GB) was quite small compared to the SSD partition (115GB) which isn't at all the normal use case, but added an extra wrinkle for testing.

To my surprise, and supporting what Bbh noted previously, it would seem you can specify slices for not just one, but both parameters.

I have a working ‘Fusion Drive’ and still have my other ‘stock’ partitions and recovery drives intact. I restored a system to the wee beastie to see if <myTestVolumeName> would boot my MBP5,4 (mid-2009)… It does; this bodes well.

So the answer AFAIK is Yes, as to preparing/preserving Boot Camp and exising partitions, at least on my setup. YMMV.

I just did this yesterday.

I was having difficulty with Bootcamp getting stuck at partitioning with my 2011 iMac with a DIY fusion 2TB+256GB. So, I disabled fusion, installed OS X on my HDD, ran Bootcamp and set up a partition for Windows on the HDD, installed Windows 8, booted from OS X thumb drive and then created a fusion drive with just the slices I wanted to use, leaving the recovery and Windows slices alone.

The Windows and recovery partitions are not visible at Option-boot until after I installed OS X again on the fusion drive.