Are there exercises I can do to improve my urine flow, even though I have bph?

No. Prostate enlaregement, or bph, occurs as a natural part of normal male aging process. Since this is controlled by hormones, there are no exercises or foods one can eat to help improve the symptoms. If a person becomes symptomatic, then medical therapy may be indicated after an initial evaluation by your physician.
All Theoretical. No studies that i know of looking at this. Theoretically you could do anything that stretches out the pelvic floor. You could try going to a pt that specializes in the pelvic (usually called women's health pt's) and see if they can give you a modified kegel exercise (what women do to strengthen their pelvic muscles) and doing this type of exercise might "pump" the prostate.