No, he can't. He* can only sign a bill passed by Congress into law.
*No, I ain't sayin he or she...political correctness is not my strong suit!
Thing I can guarantee you:
Under a next dem president (w/ D House and D senate)
1) legislative filibuster is GONE
2) In a strict party-line vote, they will pass something similar to “Merrick Garland Judicial Reform Act” that would dilute the judges and increase the SCOTUS seats
3) pack the courts using party-line vote.