At the time of this writing, it is not possible. Several tickets have been submitted over the past several years requesting this feature, presumably it is being worked on.
https://youtrack.jetbrains.com/issue/PY-33833
https://youtrack.jetbrains.com/issue/PY-20243
In the short run, if you need to convert previous projects from the default, you can find the setting is stored in the ".idea/MYPROJECT.iml" file:
<component name="PyDocumentationSettings">
<option name="myDocStringFormat" value="Google" />
</component>
By default, there is no PyDocumentationSettings component, so you'll need to add it. In Linux, you could run the following (very hackish) code in terminal to change all projects in any subdirectory:
new_docstring=' <component name="PyDocumentationSettings">\n <option name="myDocStringFormat" value="Google" />\n </component>\n</module>'
find . -type f -name '*.iml' -print0 | xargs -0 sed -i "s|</module>|$new_docstring|g"
If you simply wanted to go from e.g. Numpy to Google, you might just use:
find . -type f -name '*.iml' -print0 | xargs -0 sed -i "s|Numpy|Google|g"