600 cc implants are too big for almost anybody.
I don't know what you look like, but I have long experience with breast augmentation in New York City, and I can tell you that very big implants age very badly, stretch out the tissues, and look fake. So I don't recommend it.
But to answer your question, to put another 100cc's into your breast implants, you need another operation. Why don't you wait a while?
Overfilling Saline Breast Implants
Many women implanted with saline implants remark that they like the appearance of their breasts, when they are recently done. While they all prefer the feel and mobility of the breasts after they descend, some experience "buyer's remorse", like you and wish they had chosen larger volumes. Discuss your dissatisfaction with your surgeon. If your implants are filled to the minimum volume, there is some discretionary "overfilling" of the implants, which can be done, however it is usually in the 25-50cc range, depending on the basic volume of the implant and is specified by the manufacturer.
If you and your surgeon agree to "overfill" the implant there are several risks which include: infection, worsening of the external and/or internal (capsule) scar, damage to the implant, and dissatisfaction. When a saline implant is overfilled, it may feel firmer and while the projection may increase, the space between your breasts may widen, as the footplate becomes narrower. On the otherhand, switching to a larger size breast implant which can easily accomodate the desired 100cc volume while more expensive, is probably the most reliable method to achieve your aesthetic results. Discuss this with your surgeon.
That procedure can not be done as an office prodcedure. The bad news is that you will have to undergo another surgery to swap implants out. The good news is that it is a very easy procedure, should take less recovery time because the pocket for your implant has already been created. Give yourself one year before swapping out implants for size.
You will need another surgery.
If you want to increase your volume from 500cc saline to 600cc, you will need to have another surgery. Although saline implant does have some wiggle room to go smaller or larger, adding another 100cc to the same implant will make your breasts feel very firm and tight. It may be safe to go 25-50cc larger on your current implants. I would recommend switching your moderate-plus implants to high profile implants. Please discuss with your plastic surgeon the risks and limitations of re-operation.
Overfilling breast implants
I use Mentor implants quite frequently and I am not aware of any Moderate 500 cc implant which can be filled to 600 cc. Only the high profile 500 cc implants can be filled to 600 cc but the manufacturer describes the anterior valve as use for one time fill.
You had surgery only 6 weeks ago?
Really, just wait, enjoy yourself, go to the beach, wear dresses and bathing suits that enhance you implants. It would be a mistake to reoperate so early, for a multitude of reasons.
You may feel differently after "wearing" them for a while. Who knows, you might even be happy, or want 200 cc's more, but now is not the time. Now is the time to enjoy the new you.
Additional volume requires additional surgery
Unfortunately, you will need another full surgery to add 100 cc to your size. Saline breast implants do come with some room in terms of size, but this is usually within the range of 50 cc. You will probably need new implants if you wish to increase by as much as 100cc. If you are within a few months out from surgery, I would suggest waiting another 1-2 months before making a decision. Many women have doubts about their size initially, and then with time decide that they are happy with the size they initially chose. This is not unusual, as people tend to get used to their swollen size during the first month after surgery, and then may be surprised to see themselves become a little smaller as their swelling goes down. Good luck.
Even with the moderate profile plus implants none of them can be increased 100cc. Most have an approximately 80cc range. Also, as you fill these implants the diameter will decrease and the projection will increase. This may not be the look you want. So I would wait at least 6 months after your augmentation then decide if you still want the new implants. The exchange is a very easy operation so maybe your surgeon won't charge you as much.