TechT makes an iFit bore kit that'll extend your barrel about 2". The gains are going to be more consistency based than accuracy - if there are any noticed.
As far as an adapter goes - IF you have an Apex kit for an A5 threading... the barrel will need to screw into an A5 threaded socket. Your A5 only accepts A5 threads. Soo.. you'd need an A5->A5. You really couldn't go cocker threads without replacing your Apex kit to boot.
It may be that I'm not versed in Apex because I don't see/have a need for one, but don't aftermarket companies make barrels for it? You may be able to pick up an aftermarket ..say..J&J with the Apex slotting on the tip and have what you need, without basically having to repurchase the kit, right? Doing that, in conjunction with purchasing a cocker threaded barrel with Apex slotting may allow you to use the apex on both your Cocker threaded marker, and your A5....especially so since you can't/dont really shoot 2 markers at the same time.
So...my answer - Buy another barrel - but there may be options available to you that don't require repurchase of an Apex kit.