To Change the Isolation level:
At the Process Source Definition(option X) when the "Extract Method" is "S",
place a "Y" in the 'Update Extended Options' field.
On the "SQL Extract Options" screen see the option "Isolation Level".
The choices are CS, UR, RS and RR.
Set this field to CS or whatever you require.
Use the SAVE command on this screen to save your options to your own ISPF profile.
If run in batch or Online you can see the Isolation Level parm listed between the .DATA and the .ENDDATA lines: