There is currently no disk optimizing tool that we know of. However, it seems unlikely that you will need one, as the native file system of the BeOS, BFS, is designed to not get fragmented in the first place.
To prove it works, we recently ran some diagnostics on some systems around here that had been in constant, real world use for 6 months or more. Over 98% of the files on the three machines examined were stored in one or two contiguous runs of disk blocks.
The files that used more than that tended to be rather large (like a 50 meg South Park animation) and even those would have fairly large contiguous runs of disk blocks of 64k to 4 meg in size.
In other words, these disks did not get significantly fragmented in 6 months of heavy use. BFS is quite resilient against fragmentation.