Slide masters will provide five levels of bullets but users can add an extra four levels, totalling nine levels of text/bullet point formats.
In the Slide Master view, select the master you want to amend and click at the end of the fifth level. Press the Enter/Return key and then the Tab key. You can then write "Sixth level" or whatever text you want to write here and format this level the way you want.
Do the same as above for the seventh, eight and ninth levels of bullet points.