I am creating a single pico-8 game

http://pico-8.wikia.com/wiki/Btn

this link says

pico-8 supports button numbers just between 0-5

for me, that buttons are not enough.

I want add for example button number 6 which detects "S" for player 0

It's just example

If it is impossible, I want use key "M" or "N" as a special button so pico-8 PICO-8 could detect

" Oh it is N! Not Z or C"(Z,C,Ns button number are same)

You can get keyboard state via stat().
See ‘Mouse and Keyboard Input’ section in the manual.

I believe you're limited to the standard 6-button Pico-8 controller. If you're making a single-player game meant to be played with a keyboard, you could use the second player's buttons: S, F, E, D, (L. Shift or Tab), and (A or Q).

"keyconfig" will let you remap both players keys, just explain that a custom layout is needed

I don't think any player will want to mess with keyconfig just for 1 game... besides that suggestion won't work for BBS anyway.

so you just posted here to say "that wont work" and not contribute?
at least my solution actually gave him more buttons with any layout he might want

He contributed just fine. Telling someone that their idea isn't accessible and will most likely cause a negative experience for the user is a very valid criticism, and also, it won't work at all for the BBS which is very important to know if one doesn't.