Noob question here.. my mom is 60 yo and has 40k in CPF only in retirement account. Can I top up to her CPF by cash or transfer from my CPF to her so that she has enough to "cash out" her CPF?

You can top up your mum's RA till her FRS.
(full retirement sum which is $161,000 being age 55 in 2016)

However, you only enjoy tax relief for the first $7k cash top up per year.

You can also do CPF transfer from your OA to her RA. No tax relief but her RA earns 4% compared to your OA which is 2.5% interest.

There is no such thing as minimum amount in RA to cash out. Once reached age 65, inform CPF board your mum wish to start receiving money from her RA account n CPF board will credit her bank account monthly and accordingly. (Probably $200 per month)

Refer to the section named "CPF transfer" in:

https://www.areyouready.gov.sg/YourInfoHub/Page...

"If you wish to transfer your savings to your parents or grandparents, your CPF balances will only need to exceed the Basic Retirement Sum (BRS)—which is half the Full Retirement Sum (FRS). This is provided you have enough savings inclusive of property pledge/charge to meet at least the current FRS (if you are below 55 years old), or the FRS applicable to you (if you are 55 years old and above)."

As for cashing out, I'm not sure if your mum can cash out unless she hits FRS...

As for cash top-ups to your mum's account via RSTU:

RSTU monies are reserved monies and cannot be cashed out. It can only see the light of day in the form of CPF LIFE annuities.​​​