Hi again. I think i've understood. We seem to have a confusion of terminology in the communication. I was thrown off course by the mention of sprites - In NES graphics terminology, sprites strictly refer to the 64 hardware objects that are freely moveable on-screen. It seems like you mean importing background tiles, not sprites?
If that's the case, i believe what you ask for is already in NEXXT (and NESST). Se my example below.
First, i have this fully populated scene, just for comparing the difference: 
So, i know this image works, but let's say i want to try to make it work in 64 tiles. Then i go to [File > Import > BMP as nametable in n tiles...]
This will throw a prompt box where you can fill in any number between 0 and 256. I took the preset 64.
And here are the results:
If this wasn't what you meant, let me know!