Automatically adding user after successful login process

I was wondering whether it was possible by any means to dynamically add needed passwd/shadow information for users who have successfully authenticated through a RADIUS or TACACS server (using the appropriate PAM module), or failing that, to circumvent passwd validation by the system. I am aware I can add dynamically their home directories through mkhomedir.so but this alone is of little use as I have to add the user manually anyhow beforehand.

The goal I'm trying to achieve is:

a) Have users created automatically during the initial login (if and only if they are successfully authenticated against the RADIUS/TACACS server) on a Linux system and subsequently get a shell  (telnet, ssh, ftp etc.) (Gentoo is the flavor but I don't believe it's particularly relevant)
b) AVOID using LDAP/OpenLDAP to do that.

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