Someday (soon) we’ll have a page in the Men’s Sexual Health Forum that discusses penile anatomy/erection/ejaculation and we can link to that one source for questions like this. I’ve addressed this before in a few different threads. Here is a drawing from Gray’s Anatomy which clearly shows that the corpora cavernosa (CC) are not connected to the corpus spongiosum (CS) which ends in the expanded part known as the glans penis.
The glans expands during a normal jelq because the OK grip encircles the penis and squeezes the CS as well as the CC. Squeezing the CC alone (from the sides) won’t expand the glans because the blood has no direct path to it. The only way to expand the glans and perhaps enlarge it, is to move blood through the CS. This can be done by stroking the length of the CS, but you have to be careful not to injure the urethra (urine tube) that is enclosed within the CS. It’s quite easy to damage the urethra and have blood appear at the meatus (pee hole). Be careful if you try this.
Another way is to compress the bulb of the penis. The bulb is also seen in the illustration from Gray’s Anatomy. You can feel the bulb just below the scrotum. It is the part of the penis that is compressed during a Kegel and that causes the glans to get larger during the squeeze. Perhaps manually compressing and holding pressure on the bulb, which causes blood to move into the glans, might cause some expansion. This would have to be done while erect, of course. The perineal nerve is nearby, too much pressure can cause problems with loss of sensation (bicycle riders know about this).
Since the tissue of the glans is spongy and expandable, it might take high pressures and a long time doing this type of exercise to see any permanent increases. But for those with a concern, it’s worth a try.
EDIT: I forgot about pumping. This page shows a specially designed pump cylinder for expanding the glans. I have no experience with a cylinder like this. In theory it would work as well as manual methods.