You would have to re-do it. Though at this stage the only achievements that are achievable (Once Bohemia gets round to adding them... Shame i don't have access to add them myself) are ones implemented in the missions themselves.
The following shows the planned achievements and which ones are "done" (Ones with ** ).
I am waiting on Bohemia to add the achievements that are done to Steams Achievement System, though it could be anything from 1 days time to 5 years. Once added a patch would be released shortly afterwards (Though may take 1 day to 5 years to appear on steam).