In the <configuration> section of a plugin descriptor, you can specify <property-options>

<simple-property name="databaseType">
<property-options>
    <c:option value="mysql"/>
    <c:option value="oracle"/>
    <c:option value="postgres"/>
</property-options>
</simple-property>

The schema for <property-options> also defines an "allowCustomValues" attribute. If true, this is supposed to render a text box above the hard-coded selections to allow the user to enter a text value for a custom value (in the example above, in order for me to allow the user to select "sybase", I would need to do this:

<property-options allowCustomValue="true">
...

This allowCustomValues isn't implemented. To implement it, it requires the config UI to be coded up to support that value.