The process is pretty much the same either way, but there are some unused packets you can use.
1) Locate the graphics packet you want to modify. This is displayed in NPCustom and Fiendcrafter.
2) Find the address of that packet with a hex editor. 0x242000 + graphics packet * 3 will have a 3-byte pointer to the address of the packet. It will be little endian, so the bytes are reversed. 00 56 C4 would be 0xC45600. Subtract 0xC00000 from this, so 0x045600 for this example.
3) Decompress the packet from Temporal Flux's compression menu, using the address from above. Make a note of the compressed size.
4) Add tiles to your decompressed packet using Tile Molester/YY-Char/something along those lines.
5) Attempt to compress the modified packet back into the original space. If the compressed size is less than or equal to the previous compressed size, save it and skip to step 9. If it's larger, you'll need to find free space.
6) Check Geiger's Offsets, NA.txt for space at least equal to the new compressed size. Try adding this to the Custom Data section with Overrride Free Space set to true. If the status bar at the bottom says Warning - Free Space in Use (or something like that), try a different address.
7) Compress the modified packet at the new address and save.
8) Change the pointer from step 2 to the new address. If it's less than 0x400000, add 0xC00000 to it.
9) Locate the sprite assembly you need. The packet can be found in NPCustom or Fiendcrafter. 0x242300 + sprite assembly packet * 3. The same steps are needed. This can be skipped if you're overwriting an empty packet.
10) Check Geiger's Offsets, NA to see where the packet ends. It will be listed as (xx) Sprite Assembly, with xx being the packet number.
11) If adding, you'll need to move the sprite assembly to free space, with 40 bytes (decimal) per additional frame. The same principles for reserving space and adjusting the pointer apply.
12) Put together the frames using a sprite assembly tutorial. There are a couple floating around.
13) If you made a new packet, using NPCustom or Fiendcrafter, put together a sprite using the correct graphics.
That should cover everything. The unused graphics packets are 47, 4A, CC, and E5. The unused assembly packets are 08, 09, 0A, 8C, C5, and D8.