I suggest you send this question in a support ticket and get the straight info from the people in charge themselves instead of educated guesses from the rest of the world.

I am a moderator on the NGP site (the place where all the sprites are up for dowload),
NGP is the replacement of TBNF, who pioneered Spriting NF.

Shortly after, the EU server made it illegal to use modded sprites, whereas the US allowed it.

The only restrictions we have are:
Must not give an un-fair advantage in combat (AKA Plane sprites, Map glitch sprites, etc.

Sprites are not supported by SDE or TNF, so if you break your NF install with them, don't go to them for help.

In other words, anything in the NGP Official Releases area is legal, sound mods, harbor mods, and ship mods alike. 

what was the NGP website again :D...

as long as the sprites etc do not give u an extra advantage its allowed as lets say changing smoke sprites is illegal etc 

The only thing they lack are good solid new HUD's, although i still have my old ones that work so no worries.  Just wish NGP would work on them again since the stock one is so insanely ugly.