Can I transfer Singapore US dollars account to my singapore Singapore dollar bank account?

Its a bit annoying because funding Transferwise USD accounts from SG based accounts goes through SWIFT (unless you have a US accounts which can work through local ACH).

Whichever way you decide, the borderless account have virtual USD account details which you can use as the target of your transfer.

Once you've funded the account you can convert to SGD and credit locally; that happens via FAST so fairly quick and with only a small nominal fee.