I preferred Ed's single shot bolt action but most people don't.
I don't know what hinders it from being a possibility but some airguns like Sumatra, Rainstorm are indexed upon cocking. If your EDgun is like those in any way I can see why someone told you it can't be done.
I've literally had to use plastic tweezers or be gentle with Hemostats to load slugs in testing the above mentioned platforms.
I COULD disable the indexing system. On the Rainstorm that would be fairly easy. Though some designs are incorporated in a way that adds tension required for trigger/safety to work.
See, Brian at EDgun West maybe able to do this for you. Just know that you might loose your ability to use a multi shot magazine in the future if you choose that route.
Dedicated airgun for slugs or pellets? Very few allow all lengths & types of projectiles to be used. In all honesty I would decide if I wanted mine to send pellets or slugs, and be happy with the choice.