The "category vs text" distinction is one we are slowly phasing out, instead favouring allowing a case by case configurable decision for how to display text data. For example, the Bar view allows you to split by any text field. As such we've never introduced it into DataManager.

You should be able to convert to category in Omniscope after loading/refreshing data. If this doesn't stick when you next refresh, you may need to untick Settings > Advanced file settings > Auto-convert text fields.