MS Access - Continuous Form Select with Dropdown from Another Table - sql

Question

I've been using Databasedevelopment.co.uk's excellent example on how to do a continuous form select with a invisible button overlaying a checkbox to assign employees to a specific shift. I'd like to make it so that said continuous form also has a dropdown of the different Paycodes so that when they are selected I can use a combobox to indicate "Regular Pay, Overtime, etc....". I'm running into a wall because with the query as-is from the example, the recordset for the Paycode field is not updateable.
Messing with the primary key for the employee's table fixes the paycode issue but prevents the selection code from working properly.
I'm a bit out of my depth here, what's the easiest way to accomplish this?
SELECT CAT.EmployeeID, CAT.FirstName, CAT.LastName, ASGN_TEMP.ShiftNum, ASGN.PayCode, IIf(ASGN_TEMP.[ShiftNum] Is Null,0,-1) AS IsSelected
FROM tblEmployees AS CAT
LEFT JOIN (SELECT ASGN.EmployeeID, ASGN.ShiftNum, ASGN.PayCode FROM tblAssignedEmployees AS ASGN
WHERE ASGN.ShiftNum = Forms!frmMainMenu![txtShiftNum]) AS ASGN_TEMP
ON CAT.EmployeeID = ASGN_TEMP.EmployeeID;
Paycode is a static table with an ID, a Paycode and a description and would only correspond with each record in "tblAssignedEmployee". That is to say, there is no relationship between the employee or the shift with what Paycodes are available, I'd just like a second table for ease of updates.
---EDIT---
Table: Employees
ID
EmployeeID
Firstname
LastName
1
1234
Bob
Jones
2
9999
Mary
Sue
Table: AssignedEmployees
ID
EmployeeID
ShiftNum
PayCode
1
1234
1
OT
2
9999
2
Regular
3
1234
2
OT
Table: PayCodes
ID
PayCode
Desc
1
Regular
Regular Pay
2
OT
Overtime