I was wondering if GOLDEN Acrylic Retarder can be added to Glazing Liquid to further increase open time, or if the Glazing Liquid already contains the maximum recommended quantity of retarder in it? Is AGL, essentially, a fluid gloss medium with some retarder added to it?
Acrylic Glazing Liquid is a blend of acrylic medium, retarder and water. You can add more retarder but as it lowers the amount of medium, the film strength is reduced.

I first try to get an artist to look at the other factors, which influence working time, such as air flow, temperature, humidity, technique, absorbency of the surface, and film thickness. If they have done all they can to alter these things, and are still not able to achieve enough working time, adding more retarder is acceptable.

Adding more retarder results in a weaker film, which can be countered by applying a stronger layer of acrylic medium, such as Acrylic Glazing Liquid mixed with Soft Gel Gloss. It also can result in a tacky film that takes much longer to become dry to the touch. If you understand these things, and can work around them, then, yes, you can add more retarder.