You can convert PWE ZEN to C-Points once you have purchased the ZEN. Simply select "Star Trek Online" from the available games list, "Holodeck" from the servers list, and then the amount you wish to transfer.
Also, prepaid PWE ZEN cards may or may not be available in your area.
FYI: ZEN is the universal PWE virtual currency. The equivalent amount in individual game currency varies by game. There should be an exchange rate per game listed on the PWE Billing page.