Yes this is possible!

The trick is using a For Each Value and using a conditional decision to check if the Current Value is empty or not.

The part that does get tricky is to iterate through a multivalue field and replace the values, we will need to clear out all the values after retrieving them and add them back one at a time.

So my workflow does the following

1. Retrieve Multi Value Field Group
2. Clear Multi Value field group from the template
3. For Each Value from Step 1:
   1. Conditional to Check For Each Value_ Current Value = empty
   2. Check if this is the first iteration or subsequent iterations. 
      1. If First iteration then OVERWRITE values.
      2. If Subsequent then APPEND values
4. If the Current Value is blank, it assigns the values back with GL Amount being 0
5. If the Current Value is NOT blank, then it assigns all the values back as they were. 

I have added some screenshots and the workflow(change GLAmountTest.txt to GLAmountTest.wfi) for you to take a look at. If you have any questions please let me know!