HaHa! Got it! You can do it

In reply to Adobe Opentype Univers Roman set to metrics…:

HaHa! Got it! You can do it with probably many glyphs/classes, as far as you keep them in three groups and one glyph/class (middle) is affected:
pos o f space' T w o 300;
or even more than one glyph/class:
position s f'10 t' -5 period;
where in sequence s-f-t-. the x-advance of f is increased by 10, and the x-advance of t is decreased by 5.