Ok, it seems like some inner hack ;-). But I believe it should be easy to add it.
Open Doctrine2.skipper.cfg.xml file available in your instalation directory (or create separate new configuration file as described here https://help.skipper18.com/expert-usage/customization/configuration-files/ )
and add following element
<attribute name="jsonb" type="true" help-text="" />
to
skipper-configuration / orm-configuration / attribute-types / struct[name="field"] / struct[name="options"]
it should be on line 130 in mentioned cfg file.
Final result should look something like this:
<struct name="options" help-text="" help-url="http://doctrine-orm.readthedocs.io/en/latest/reference/annotations-reference.html#column">
<attribute name="jsonb" type="true" help-text="" />
<attribute name="fixed" type="bool" help-text="Boolean value to determine if the specified length of a string column should be fixed or varying (applies only for string/binary column and might not be supported by all vendors)." />
<attribute name="comment" type="string" help-text="The comment of the column in the schema (might not be supported by all vendors)."/>
<attribute name="collation" type="string" help-text="The collation of the column (only supported by Drizzle, Mysql, PostgreSQL>=9.1, Sqlite and SQLServer)."/>
<attribute name="check" type="string" help-text="Adds a check constraint type to the column (might not be supported by all vendors)"/>
<attribute name="version" type="string" help-text=""/>
</struct>