Unfortunately, the answer to this question is „No.“ The Financial Industry Regulatory Authority or FINRA (previously the National Association of Securities Dealers or NASD) requires an individual to be sponsored by a member firm, a self-regulatory organization (SRO), or an exchange in order to write the Series 6 exam.
The Series 6 exam allows a finance professional to demonstrate his or her competence within the area of mutual funds and variable annuities. In order to schedule an exam, the member firm that sponsors you must file the Uniform Application for Securities Industry Registration or Form U-4 with FINRA.
For Form U-4 to be completed to the liking of FINRA, the sponsoring firm must act as an Appropriate Signatory. Therefore, if an individual attempted to file this form without the signature of a sponsoring firm, the application would be rejected and monies paid toward registration might not be refunded.