Too small shoulders can only be fixed on jackets across the back.  That means: if you need just a little more room to eat a burger: Yes.  If your shoulders are pushing out into your sleeves: No.  And even then, this only works if the original manufacturer has left enough seam allowance.  Typically, you should be able to add an inch (maybe an inch and a quarter) across the back shoulders.

While the back shoulders can be taken out, taking them in is far more complicated, unless you have exceedingly erect posture -- like ballet dancer with a tall wig kind of good posture.  This occurs because good jackets are cut along the back with a slight curve that follows the natural arch of the spine - convex at the shoulders and concave at the small of the back.  Taking out a jacket at the height of the convex curve only makes the curve slightly rounder.  Taking a jacket in at the convex curve just eliminates the curve, hence, only if you very upright posture and minimal upper back arch.  Taking in a jacket here without this back shaping makes a concave arch of a concave one and makes a mediocre jacket only worse.

Adjusting a suit at the shoulder seam - the length from the neck to the armhole - isn't ideal in either direction.  You can probably get an extra quarter of an inch on each shoulder, but that involves taking off the sleeve entirely and can get expensive.  Shortening the shoulder seam involves making a dart and will leave a rather visible seam in the front of your jacket.  If  your jacket has a canvas, this can't be done at all.



Really can't be tailored at the shoulders without getting a little funky.  Talk to your tailor or HRD if you're really desperate, but you're probably better off just buying another shirt.  If you consistently have trouble finding shirts that fit in the shoulders and neck, custom shirts may be your only option.  Fortunately, there are lots of good mail order services and Your Dapper is hoping to expand into shirting in the near future (so, you should buy lots of bowties so we can get on this).

In short, shoulders are really not the place to be making adjustments to your clothing.  Most tailors will only take out the center back seam of your jacket, and only if the manufacturer has left enough seam allowance.  The neck and shoulders are the most important places to fit when buying.