Yes, you can backup the configs/data from an AIM and restore it to an NM. The only thing I would watch out for is if in the existing config (show run) you have limits placed in the configuration (on the number of ports, voice mailbox size, etc) that would then be restored to the NM. For example, if you have a mailbox created with a size of X (probably the default for an AIM), then it won't automatically get the new default mailbox size when you restore it to the NM but will rather retain the old mailbox size. None of that will prevent the restore from working, but if you're paying for an NM, you might as well have it use the system to its capacity. All this can (and should) be fixed after the restore on the NM. The voicemail limits are documented here:
http://www.cisco.com/en/US/products/sw/voicesw/ps5520/products_tech_note09186a008036fd99.shtml
There are some other things to look out for. If you look at a 'show run' you'll see a bunch of secions that start out as "ccn trigger ...". In there you'll have something like "maxsessions 4". You'll usually want to increase that to take advantage of the fact that the NM can support more sessions. The other maxsessions parameter is under the "ccn trigger..." section.
As long as you clean up those areas, you'll be fine.
