Since OTRS 2.0 it is possible to de- and encrypt messages with PGP. Also you can sign outgoing messages. Before this feature can be used you need to activate it and change some config parameters e.g. with the graphical configuration frontend to the needs of your operating system.
Through the "PGP" link in the admin area of OTRS it is possible to manage the key ring of the userer who shall be used for PGP with OTRS, e.g. the local OTRS user or the web server user. It is possible to add and remove keys and signatures and you can search through all data in your key ring.