I assume you're using Riivolution? You can do a pretty simple memory hack:
Just stick this in your Riivolution patch. It covers all versions of NSMBW (besides the Nvidia Shield version, lol). Naturally, you'll want to replace the value with what you want your save's filename to be, converted from ASCII to hex. It also has to match the number of characters as the original ("wiimj2d"), so, 7.

You can optionally also move it to the SD card* instead of keeping it in Wii internal memory alongside NSMBW's, so that copying/moving/deleting NSMBW's save file in the Wii Menu doesn't affect your mod's save (and so that it is more easily-accessible).
Again, obviously, change the value of "external" to wherever you want it to reside on your SD card.

*Moving the save file to the SD card is not necessarily a good idea. A large portion of SD cards, for some reason, refuse to be written to by Riivolution. So the game crashes when it tries to create the save file, for a fairly large portion of players. Do it at the risk of having to instruct people to remove this line from the XML once in a while when it doesn't work for them .

Hope this helped!

So, to rephrase: pick a seven-character name for your save file (ex. wiimj2d, NerSMBW, NewerDX, etc). Then put it into this website (set output delimiter string to "none"). Then copy the hex output; that's what replaces the save name.

Be sure to replace the path in the second patch I sent as well, if you choose to use it. Here's a practical example: