the parts available on this site can be used with any editable NFS. unfotunately nfs:underground is not editable at this time but there is some one by the name of Arushan who is working hard to crack the format and create some programs to edit it.
Thats great, I think EA are stupid to not have thought of people wanting to add their own parts, its pretty obvious that everyone will get bored with default parts eventually and want their own, cant wait.