The hoses on my diaphragm pumps attach by sliding directly over the outlets with a spring clamp holding them in place.

They've come loose a couple of times, so I'd like a more secure connection.

Would it be possible/advisable to cut a thread on the outlets and connect hoses with standard pipe fittings?

NPT 3/8" looks like a good candidate -
the outer diameter is 0.675" (17.145mm), only slightly less than the outlet diameter (approx 18mm).

I would not thread the outlet pipe directly.
I would use an adaptor like a glue on/hose clamp style that adapts the smooth pipe to threads.

The problem is that the pump nozzle is made from HDPE which requires special adhesives.
I'm not sure these adhesives can make a durable airtight seal between the nozzle and brass or nylon fittings.

Having said that, I'm starting to think this is the way to go.

Solution one might be a worm gear type clamp.

What I used on mine was a section of more flexible hose from the pump to the manifold. It is softer vinyl hose and clamps better to the smooth outlet. If you don't use a manifold, you could still use a short section of vinyl or EPDM hose with a butt connector (or tee, good place for a low pressure pressure gauge) and worm gear hose clamps.

I'm currently using a worm drive clamp, but the hose still works itself free every few days.

I haven't been able to find any vinyl hose in the right size.
I suspect it will grip even worse than the existing hose though!

Maybe rough up the surface for a better grip? Or cut a small channel for a clamp to sit in.

I would not thread it!
It sounds like more of an issue with the hose selection to me.
You should be using something like a "5/8' reinforced flexible clear hose". Local hardware store should have it. It will go on tight, use alcohol to lubricate the the interference fit.

I purchased a pvc 3/4 nozzle fitting for mine and used the pvc primer/glue. Airtight and secure

Hey Damian,
I just put my aeration system in and contrary to what I've read here, my pump (Gast Diaphram - Model # DOA-P701-AA) came with both the outlet and inlet threaded for 3/8npt. Barbed fitting and air filter were included (only needed some Teflon wrap) as well as the stainless worm clamps to attach the 3/8 sinking hose. Its only got a few hours on it, but so far...no issues, and that little 1/8hp pump puts out some serious air!!

So apparently it can be done. I'd be very careful not to crack the outlet tho.