Yeah, if you have a celeb in the household, none of the household members can get an autograph as the option won't be there. But there is a way that I found so one of my celebrities could have a complete collection of autographs that includes their own. It's not that hard to do but it takes a bit of time. This is how I did it.
Play the celebrity household and have them meet a sim in another household and become friends. Doing that makes it so the sim you become friends with has the full social menu and you don't have to go through the hassle of repeatedly trying to meet the celeb which can take a long time. Then switch to the household of the sim you just became friends with. Have them socialize with the celebrity somewhere and ask for an autograph. Once you have the autograph in the sim's inventory, immediately gift it back to the celebrity. That's why you need to have the full social menu so you get the gift option. Then switch back to the celebrity's household and you should find the autograph in the celebrity's inventory.
Hope this helps.
