The ‘runas’ command will not accept the password as a command line parameter, but you could use BigFix’s native ‘override’ options for the ‘wait’ command to run as the user.
Tip - Action Override User settings has several examples. For this case I’d probably choose examples 9 or 10, where the BigFix Operator is prompted for the password when they use the Take Action button from the Console.
Example 4 might also work, but would require embedding the password into the ActionScript, which is not stored securely and might be accessible to other machines that can reach your Relay/Root.