That's a bit hacky, normally the children are the ones who define the alignment inside the parent manager..
You create the hfm using all width available:
HorizontalFieldManager hfm = new HorizontalFieldManager(Field.USE_ALL_WIDTH);
Then you add the fields (or managers) aligned to left:
hfm.add(new LabelField("Left", Field.FIELD_LEFT));
And then you add the fields that must be aligned right. I'm using labels here, but every class extending Field (including managers) has a constructor accepting flags as argument:
hfm.add(new LabelField("Right", DrawStyle.RIGHT | Field.USE_ALL_WIDTH));