No, you won't be able to import a 3.0 backup into a 3.3 server, it'll error out on you. The database format changes quite significantly from version to version with all the new features, so the format of the 3.0 backup will not work in a 3.3 import.
The way to do this is to set up your Win2K server and install 3.0 on it first. You can then either backup and import the database onto it, or set it up as a replication client and replicate the database over from the NT server. Once the 3.0 database has been imported/replicated onto the Win2K server, then upgrade this server to 3.3 (just install 3.3 right on top of 3.0), the installation process will ask you if you want to keep your existing data, choose yes and it will convert the database for you.
Note that to upgrade straight to 3.3 you need to be running at least 3.0(4) as seen here:
http://www.cisco.com/univercd/cc/td/doc/product/access/acs_soft/csacs4nt/acs33/winrn331.htm#wp1064339
Once you've got the new Win2K box up and running you're good to go. If you don't want to change all your NAS's to point to the new servers IP address, you can change the Win2K servers IP address to that of the old NT box by following the instructions here:
http://www.cisco.com/univercd/cc/td/doc/product/access/acs_soft/csacs4nt/acs31/acsinst/install.htm#650586
