I think you have terminology mixed up. First off, the various CGC labels are explained here-
https://www.cgccomics.com/labels/
CGC has a Signature Series (yellow labels) that they will both grade and certify a signature as authentic, but they have to WITNESS the signing. So your Stan Lee books are not eligible for Signature Series (yellow label). To learn more about the Signature Series, go here-
As for your books-
Yes, CGC will still grade them. HOWEVER, since they can't verify or authenticate your signatures, your books will most likely get a Qualified (green) label. To see some examples of what that a signed book with a CGC green label might look like, go here-
[UNLESS the signature is on the inside of the book and not on the cover. Because that technically doesn't downgrade the condition of the book, you would get a Universal (blue) label, where it says "Stan Lee written in marker on page 1". But even that is tricky, since CGC isn't verifying the signature - it's just saying that it's there.]
So unfortunately, your options here are pick-your-poison-
If you go through CGC for your books, the green label is your only option - but for the purpose of selling, you'll probably take a hit because green labels aren't as popular. Especially since Stan Lee is still making appearances and signing books that get Signature Series (yellow) labels. Or you can go with one of CGC's competitors who offer grading and/or signature verification, but then you'll take a hit because you won't be selling a CGC graded book anymore. Or you can bypass grading and see what you can get selling your books raw/unslabbed.
Hope this helps!