Sure, just use one of the punchdown splice blocks. It's less than ideal if the splice is in an inaccessible area or outdoors. The splice blocks I know of are not weatherproof.
Each connection adds to the potential failures of a line as well as noise on a line and distance that you can run the line. Ideally you want to keep the number of connections down low.
If you have to do it, do it carefully and test the connection and be certain it is secure from being pulled apart and protected from moister.
In my experience with doing so as you are asking I say go ahead, IP cameras and POE are a little more forgiving than say splicing a DATA backbone cable. In some of my jobs, contractors were to run the cables and have come up short a few times and I had to either splice or run a new cable. I spliced with zero issues however none of my work was outside. And yes as mat said every splice is a potential issue in the future. Make sure it is sealed well.