First P (0 indent) is a separate item and has no requirement for or against Justify (and excluding it, might get tricky for bulk/generic conversions)
To set the default for NEW, first time, conversions you set this in:
Preferences: Common options:
And (or if converting again. In the dialog at the begining of Convert)
Look and Feel: Styling options (tab):
Extra CSS:
Code:
p {text-align: justify)