SQL insert

Hey im trying to insert a value into a table Ledger where the field is null.
The value i want to put in is stored in a table called tblPatient and the field is called LocID ledger has a foreign Key PatID that connnects to tblPatients Primary Key [MedicalID#]. Is it possible to do somthing similar to the following ? or is there a different/ better way to do this ?

INSERT INTO Ledger ("Location") VALUES (Select LocID FROM tblPatient WHERE tblPatient.[MedicalID#] = ledger.PatID) WHERE Ledger.Location IS NULL