A WFA workflow goes through a planning phase where all the command inputs are resolved . This is followed by an execution phase in which the all commands code is executed.
Add-WfaWorkflowParameter happens during the execution phase of the workflow . This is when your CLI is executed. While row coditions are evaluated before during the planning phase. So what you are asking is not possible.
But this can be achieved by writing code like this in the beginning of your command.
$executeCommand = Get-WfaWorkflowParameter -Name executeCommand
if (!$executeCommand) {
return;
}