You are free to add the dental care option to your plan at any time. If you initially decide to forego this option, and then apply for coverage at a later date, there is no penalty or restriction period before you can claim full benefits. The only requirement is that you remain enrolled for at least 12 months after you've added dental care to your plan.
Please note if you decide to cancel your dental coverage after the minimum 12 month requirement, you must wait 24 months before you can repurchase this benefit.