To get v21 posting exactly as you want, you will need to learn about scripts.
Select Modify for the Comp Left/Right menus and see if there is a @l@ G40 in the Chain End window.
Are you using the Machine > Profile command to create your code?
Otherwise, you would need to manually add a straight segment to the chain.
A script could possibly be written to do this, but it could be pretty involved.
Now you can either post the new code directly into the empty spot(not always a good way) or post it at the end of the program and then cut/paste it into the right location.
Always make sure the tool retracts to the Z rapid plane when making edits.