You can't use Social Security funds to fund a Basic Income. It operates similarly to a pension fund so it would be equivalent to confiscate peoples lifetime savings. Good luck explaining that to someone who is just about to retire.
Yes, you can.
> It operates similarly to a pension fund
It presents itself as one, but it really operates as a general-tax-supported social-benefit program with a variety of eligibility criteria.
> so it would be equivalent to confiscate peoples lifetime savings.
Sure, some people would politically attack it that way, particularly if the general BI reduced the SS benefits to which people already qualified would receive. Its less subject to this political attack, however, if SS is merged into BI in such a way that no one currently paid into SS ends up with less BI at any point than they would have received from SS.
For the record, you have zero ownership rights in your SS benefits. They can be changed at will.