Hmmm... that is not as trivial as it sounds. You have to realize, that, coming from DAZ3D Genesis 2, male and female models are something very different. They are using different UV-maps (skin, textures), different clothings, hair..., and different morphs. Some vew versions ago, it was even (almost) not possible to simply use female morphs on male models. Before that, it was much work to "convert" female to futa. This is a bit less complicated, now we are able to use female morphs on male, but don't think of some kind of simply convert it. Better think of something like "recreate" a female look on a male figure.
This will never be easy to do or will have the identical look. No "one click" thing, no "simply use penis on female".
First thing to do is to prepare the male model: Open it by selecting a futa skin and then make it look like the base female figure by dialing out all the male specific morphs and by dialing in the (don't know the name from memory) G2 female shape for male. You can also try to load one of the ready made basic build-in looks for futa as a starting point.
After that, dial in the morphs you have used on your custom female model, to make it look identically. (On top right of the morph menu tab, you can choose to "use female morph on male" and vice versa).
It sounds lame, but that is how it is to be done. Because female and male morphs are different in theory, just mergin models will most likely fail. You can try to save your custom female's look as a morph-preset and/or to merge all the morphs with Morph Merger in advance, to make it a bit more easy for you. But anyway, you will have to do some amount of editing after that.
