I have DBS Multi Currency Account with USD; can I deposit USD into TD Singapore account instead of depositing with SGD?

No, you cannot - the TD Ameritrade Singapore account with DBS only accepts SGD. They will then convert it to USD for you.

However, not to worry - you can transfer your USD to TD Ameritrade's US account using DBS Remit, which is free if you already have USD in your account. Once you have transferred all the USD in you account, it will probably be better to transfer SGD to their SG account because the rate that DBS provides TD Ameritrade is better than the rate they offer to normal folks like us.