Can, but then you won't be able to claim 2016 tax relief unless you put in 12k.

Best way to 'play' is probably to put 6k per year for a few years until its sizable amount (let's say you go for 5 years), then withdraw one short in the 6th year. 7th year start putting 6k again. This will result in much better interest rate (just put the withdrawn amount in some investment or long-term FD) IMO compared to every year just add and add.