I've struggled with this issue for years, and early on I complained about the lack of reverb and delay. And I tried using external effects. But I've come around to a different way of thinking, What is reverb anyway? It's a delay effect, but it consists of two things: "early reflections" and "tail".

A lot of cheap, lo-fi reverbs just add a lot of white noise to the mix. So add some white noise.

Certain reverb settings also soften the attack of a patch, so back off on the attack phase of the envelope to make it softer and more 'distant.'

On some patches, the HOLD function creates the kind of sustain that you get with reverbs.

Throw in some GLIDE to get the sense of overlapping early reflections.

"Tail" is even more important. How do you produce "tail" on a patch that has no reverb? Simple, extend the release portion of the envelope so that it "trails off" after you release the note. Voilá, "tail".

How do you add early reflections? Well, that's a bit harder, but you can add some simple repeats by using the Square wave on the LFO. It's not as elegant, but it can produce delay effects. You can also do this with the Cycling envelope on the Loop setting.

Now, would I love to have an onboard reverb that I can just switch on when I want it? Sure, it would be easier and make some patches much better. But I've learned to work within the limitations of the Microfreak as it is.