...That is what I did... and worked fawlessy. The only drawback is that the user will not show up on the login screen as a list, since only UID>=1000 are shown. Changing login.defs as I said, the user will show up but I think that, since Ubuntu system users have UID>=500, eventually system users will show up a day too - that is what I'd like to avoid.

For now I've changed the user UID and the login.defs file and is still working. If there is a way for doing better, I'll surely switch to that solution.