Set a fine sieve over a small saucepan. Working over the sieve, scoop the seedy pulp from 5 of the passion fruits. Press on the solids to extract as much juice as possible. Add the sugar to the juice in the saucepan and cook over moderate heat, stirring to dissolve the sugar, about 2 minutes. Transfer the passion fruit syrup to a medium bowl and stir in the lemon juice. Let cool. Scoop out the seedy pulp from the remaining passion fruit into the syrup and stir.