ONTAP expects the user and group identification information of a new volume as an integer value. The default value is 0 ('root'), and this representation is similar to how an user and group id is represented on UNIX system.
You can change the $userid parameter type from Number to String in Workflow UserInputs or if your input parameter is really a string and is specified in UID:GID format, then you can use some function to split the IDs and can return an integer value of it. As an example, you can refer the certified 'splitByDelimiter' function.
OK, I think you want to provide name instead of integer value for a volume user or group ID, and netither WFA nor OCUM cache this name/id information from ONTAP.
I don't think changing dictionary entry / command would solve this problem.
To workaround this, we should first get the existing user name to id mapping information from ONTAP using 'unix-user show -vserver' CLI command, and then provide that mapping information in the workflow user input query for $userid parameter.
e.g.: In your workflow, change the $userid parameter type from String to Query and provide the name to ID mapping information in the query like shown below. This way you will always know the name and id for the user/group while creating the volume.