PLEX is for everyone
PLEX can be used by players regardless of the type of account they have. When you activate a PLEX, the extra time is added to your account immediately. In this respect, Eve plex works exactly like a prepaid game time code.
If you are currently using a EVE PLEX, your account is converted to a paying account and the 30 days are added onto the remaining days left in your trial period.
If you have a recurring subscription, the 30 days are added onto your account and billing resumes as normal after that time.
If you use game time codes to pay for EVE, PLEX adds days to your game time in exactly the same way.