Yes, in several ways:
1. Starting on your 55th birthday, you can make voluntary contributions to your Retirement Account, up to the Enhanced Retirement Sum (ERS).
2. You can make voluntary contributions to your Medisave Account at any age, but those contributions must live within two limits: the CPF Annual Limit (currently $37,740) and the Basic Healthcare Sum (BHS, which will be $54,500 in 2018).
3. You can make "all 3 account" voluntary top-ups, again subject to the CPF Annual Limit. Exactly how those funds get allocated to your sub-accounts depends on your circumstances. For example, if your Medisave Account is already at the BHS, then the portion of your "all 3" top-up normally allocated to MA will spill over into another subaccount. (And above 65 the MA portion is 84%, so there's a lot of potential spillover available.)
Yes, this is quite strange. Essentially you can deposit up to $37,740 every year in a 2.5+% on demand savings account, assuming you've already topped out your restricted funds (MA and CPF LIFE). Well played if you can do that.
4. You can top up any CPF member's account(s), subject to the
recipient's applicable limits at his/her age.
No, that's incorrect. You can delay your CPF LIFE payout start date to age 70. And you should if you don't need the money and are in at least reasonably good health. Only CPF LIFE payouts must start at age 70. Your other CPF funds can stay in place as long as you want, for your lifetime and even a bit beyond.
One exception: MediShield Life and (if applicable) Integrated Shield base plan premiums must be deducted from
somebody's Medisave Account, up to the withdrawal limit for your age. Your Medisave or "all 3" contributions can/will replenish those funds. You are not required to spend Medisave funds otherwise. It's generally a good idea for the CPF member in the household who's in the highest tax bracket to assume MediShield Life/Integrated Shield premiums for the whole household, in order to maximize tax relief. Or, alternatively, somebody with a zero Medisave balance can assume those premiums, and then they can be paid in cash.
If you cannot find anything to spend those age 70+ CPF LIFE dollars on, no problem. Just send the money to me, and that will solve your problem.
Is this a "CPF hack"? Yes.
Assuming you max out Medisave and participate in CPF LIFE at least at FRS level, starting from age 55 you're allowed to deposit up to $37,740 per year (including compulsory contributions, if any) into the equivalent of an "on demand" savings account earning 2.5+% interest. That's a great deal, and congratulations if you're able to do that. If you're approaching age 55 -- even years away -- then this aspect of CPF is better than a fixed deposit, SSB, or even medium term bond. So yeah, go for it.