It can be done and it's actually easy to do...Beckson makes retrofit kits to add inspection ports to PE tanks. No "glue"...use a hole saw to create the right size hole...the flange is secured using screws. A rubber gasket creates the seal. A rubber o-ring on the cap seals it. All of this is included in the kit. O-ring should be replaced every few years.
Google "Beckson holding tank inspection ports"...you'll find 'em at retailers all over the place, even Amazon. SeaLand has them under their own brand name, but for a lot higher price than you can find as Beckson's own brand name.
Why would adding an inspection port make any difference if the vent should clog (which won't happen if you maintain the vent)?