Is there a way to umask only a specific directory?
I want that everyone who add a NEW file in this folder gets its file moded 700. But that adding it in any other folder still moded as default.

Excuse my poor English! I'm not a native speaker
Thanks in advance

Can the users add files from a command prompt, or are the files created as a result of running a program or script?

Both(?)
I've got people acceding the folder and creating files locally and via samba network.

The files saved though samba can be set to 0700 by setting the mask in the samba configuration file (/etc/samba/smb.conf ?).
For users with access to a command prompt, I suppose you could make their .profile read only, set their umask to 0700 and move chmod to a directory outside of their PATH
Set the sticky bit in the parent folder (chmod +t directory_name), so that only the owner of the directory can delete it.

I've solved using access control lists. man setaclf

Set read only perms to any new file created in mypreciousdir by a member of the riskyusers group.