Hello Giuse!
Thank you for your support,
If I remember correctly I was able to change alter the table itself but still wasn't able to input the needed expression, I also tried to write it directly into the sql but didn't work.
Today I think it is a problem of seblod itself, I find a lot of troubles trying to create a functional multi-stage form, the worst part is that sometimes it works and the next day it doesn't work anymore.
Thank you anyway, I really appreciate a reply in this lonely and deserted land :)
Oh wait, nevermind, I see that you're from Italy too! Cheers!