Can I add more filler into my filled lips? (Photo)

From your picture it looks like your lips have adequate volume. I would not go injecting any more, especially to the upper lip alone. Remember, when enhancing the lips with filler you always want to maintain the natural anatomy of the lips. The bottom lip should always be slightly fuller than the top lip. Adding more to your top lip will risk giving you the "duck lip" appearance. 

It is important to have lips look natural. Adding more especially to the upper lip may cause the upper lip to look more pouty and unnatural.In my practice there are always concerns about having lips look unsightly. Always look natural!!!

From an aesthetic point of view, I would recommend you do NOT add any more filler because your lips look adequately full and lush, and it is possible if you add more they will start to look unnatural and out of proportion with your other delicate features.  Beauty involves balance, so when lips are overdone it throws the facial beauty balance off.  From a safety standpoint, filler in the lips nowadays is most likely a form of hyaluronic acid or silicon.  Either can be repeated at anytime.  Mixing brand names of the same ingredient of hyaluronic acid is fine. Adding hyaluronic acid where this is silicon or vice versa is also fine.  Good Luck !!

It is very important to use your natural structure as a starting point when performing injections. People want to look enhanced, not altered. Injecting only into the top lip will definitely give you an altered, unnatural look. Most likely, based on the already full size of your lips, it will also give you a "trout pout" or "duck bill". It's fine to mix products together, and most likely you have a hyaluronic acid filler in there now. But really, look at the aesthetics of everything first, before you add more. "This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."