A trial can only be used to create a new account. Once you've added a retail code, it's too late to use a trial code.
I think there was a Going Rogue trial at one point; not sure if it's still available. My understanding was that it could be used like a normal trial, or could be applied to an existing account to let you experience GR for the duration (10 or 14 days). It didn't add time if used on an existing account, just opened Praetoria and the new powersets. Anything that required GR was locked out and inaccessible until you upgraded, once the trial ended.
@Roderick
Thankyou.
I am fairly sure I got free time one-time-only with a trial. But, that could be ages ago and this may work a lot differently.
Anyhow ty again I will take your word for it. 
Looking back over my account I see that I used a trial account activation to add time to my main account. Will this still work on a different account? (I know I was only able to do it once on the first.)
If so... would this trial adding time also let the player try out the Praetoria content?
ty!