Are you looking to "extend" it?
Just get some hook and loop (Velcro), put it together by overlapping it (versus mating both sides entirely). You essentially have made an extender that will allow the collar/neck connection to be expanded.
If you want to get fancy, you can sew it to ends of nylon strap, or stitch the pieces of opposed Velcro together, butt joint style.
Velcro comes in different strengths of adhesion. This is dictated by the aggressiveness of the plastic hook portion, at least in clothing applications. For industrial use, the loop portion can look very similar to the hook side (like on your windshield toll transponders).
I really don't think I would bother or trust glue to do the job. This is a great time to acquire some button/carpet thread and stout needles from MallWart. Using a pair of needle nose pliers, just stitch it on. You can glue it on to position it, I have even used Goop brand.
This works well on BC's, but you have to be smart enough to avoid the bladder. Either way, your likely to stab yourself with a needle if you do this as long as I have... 40 years or so
