you can use the LIB2NIST library conversion tool. They have it for free online (https://chemdata.nist.gov/mass-spc/ms-search/Library_conversion_tool.html) and it usually comes with the NIST library as well.
LIB2NIST can handle a certain amount of spectra to handle at a time. I don't remember the exact number. Please split the spectra something like 10k for each. And use LIB2NIST.