The drop down does not need to be connected to an enum. This can be connected to the parameters defined in your data set. So for example the report is about persons that have a relation to an organisation, the dropdown could be connected to the organisation parameter. Now the user can select the organisation and filter the report by this parameter. The organisation for example has a name stored in a text box and you can use this value in a drop down field although it is not an enum field that is used for the data. Having a text field for the user to type information is I think not supported.