How to allow single user to use user/pass authentication in openvpn when all others will use private keys?

I've got a Mikrotik Routerboard RB750GL which I bought to configure as an openvpn gateway for lan at the office. Unfortunately I was lured by the openvpn promise, I didn't read to carefully to know, that it only supports user/password authentication... Anyway, I have to deal with that. My idea is to have a LAN where every client will be on our vpn network + to have clients using software vpn clients who can connect from anywhere using their private keys. I'm not sure openvpn can be configured to use both,from what I understand it's one way or the other, is that right?

A sample/example of ovpn server configuration to handle both user authentication methods would be welcome.