Yes, fortunately you can perform that transfer online, and I believe it's instant. Go to this page:
https://www.cpf.gov.sg/members/services/forms-and-e-app
then expand the Retirement section, then the Retirement Sum Topping-Up Scheme section. Look for the entry "Top Up Retirement Sum Using CPF (For Members age 55 and above) - CPF transfer to my RA," and click on that entry's "Apply Online" link.
Try doing this with $10 now and, if you see the $10 appear in your RA when you run an online statement report, do it again for the full amount you want to transfer. As long as your SA is still zero (your proceeds from the CPF Investment Scheme-SA haven't been deposited yet), and as long as the transfer is instant, you should be OK.
Bear in mind that 2.5% interest earning dollars are still very good dollars. If you have
gobs of cash lying about and doing nothing or almost nothing, you'll probably want to use cash to make RA top ups.