I assume that you want to open a ticket when a notification arrives (and potentially close it when a RECOVERY) notification is sent.
I don't know OTRS specifically, but most service desks allow you to open a ticket via email. Nagios is very flexible so there are a number of ways you could approach this (I'm sure others will add to this list):
1) Configure a specific notification command that you can use just for your help desk. Have Nagios execute this command as well as the regular one.
2) Create an event handler (can be nothing more than a script) that Nagios executes. For example, you could create a template with substitution parameters, use a script to fill them in, and use wget or curl to send it to OTRS if it has a Web API.
3) Write an event broker which would be like using an a-bomb to kill a flea
I just spent a few seconds Googling OTRS, and they do indeed allow tickets to be open via email.