Transact SQL :: How To Update Subsequent Columns If Proceeding Column Is Not Null

I've got a table with 6 fields :

EmployeeAccess
(MasterID, LoginID, AccessID, Storage1, Storage2, Storage3)
that needs to be updated using the data in the following spreadsheet
NewEmployeeAccessData
(ID, MasterID, AccessID1, LoginID1)

There is a 1:1 relationship between the two tables..I'm trying to code a pair of update statements on the EmployeeAccess table (1 for LoginID, 1 for AccessID) with the following logic:

If LoginID is NULL, then Update LoginID with new LoginID1 value,
If LoginID is not null, and Storage1 is NULL then Update Storage1 with New LoginID1 values
If LoginID is not null, and Storage1 is not NULL and Storage2 is NULL then Update Storage2 with New LoginID1 values
etc etc...

The same applies when trying to populate the AccessID column

If AccessID is NULL, then Update AccessID with new AccessID1 value,
If AccessID is not null, and Storage1 is NULL then Update Storage1 with New AccessID1 values
If AccessID is not null, and Storage1 is not NULL and Storage2 is NULL then Update Storage2 with New AccessID1 values
etc etc.

I have no control over the schema of this table  so I'm trying to work the logic on how to update the columns in my table only if the corresponding column data is NULL, else update the next non NULL Storage column.