Can you fix a brand new J Bead chanel liner that has slipped off the pool wall without draining the pool if the liner is 18 x 30 x 54?

no, you need to drain the pool to get enough liner to pull over the top. You can try to glue a new piece to where it has pulled away but the weight of the water will most likely pull it apart and if not you will have a sag there sooner or later. When installing a new liner NEVER! trim the excess away without leaving 12 to 18 inches extra, water is heavy and the ground under the pool is sure to settle some. Some areas will stretch a lot and you will need to let it out and others will sag and you will need to take it up so plan for it. Most liners will have wrinkles in the bottom and that is where you will need to get your extra from, if not then a new liner may be necessary.