Can I add NumLock to my Windows Startup folder?

Yes. This is one of the most popular uses for NumLock. Press Win+R, type shell:startup, and press Enter. Create a shortcut to NumLock.exe in this folder with your desired parameter (for example, target: C:\Path\To\NumLock.exe on). Every time you log in, NumLock will set the Num Lock key to your preferred state.