In general: no, you cannot convert from one filesystem type to another.
In practice you can, but the two filesystem types have to be very closely related. For example, ext2 to ext3 is possible, and you can in fact mount ext3 as ext2 if you have to. This is because ext3 is basically ext2 with journalling, but apart from that the on-disk structures are essentially the same.
Specifically: the only way I know of to go from NTFS to $(not NTFS) is to back up the partition and reformat, then restore. If you have enough free disk space you might be able to shrink the partition, create a new one in the free space, copy, then delete the original, then grow the new one. This is all theoretically possible but can fail in exciting data-destroying ways.
I'm curious about your use of NTFS under linux. Is this because you are expecting to use the same partition under a Windows flavor that speaks it? If not... what's the point?
As far as ZFS goes, I'd personally wait before playing with it on Linux a while yet. Of course, I'm hideously conservative. On the other hand, you appear happy with NTFS under linux, and I'd rate that about the same risk.
The reason for my caution is because both NTFS and ZFS are filesystems which were invented elsewhere. While NTFS has been around a long time, Microsoft has fiddled with it and the last time I looked at NTFS support the read/write support was "usually works but good luck to you anyways". ZFS is a little more open, being in OpenSolaris and all, but it still isn't invented by the authors of the Linux support so whenever there is a problem they get to wonder if it's a flaw in the original specification or a flaw in their implementation. ZFS is neat, no question about it, but my impression is that the Linux support isn't quite there yet.