In order to be accepted by maketarget, a mesh has to have the same number of vertices, in the same order, as the maketarget meshes. You may be able to create a target from an exported human if you check "helper geometry" when exporting, to get the same geometry seen in the "fit tools"/helpers, and even then I'm not sure if it will work.
A reliable way to make a new target from an existing file is to download the makehuman source code in order to get access to the official targets used in makehuman. Then you can load target file, and combine official targets however you like (found in makehuman/data/targets of the source code).
https://bitbucket.org/MakeHuman/makehuman/downloads/Another method of creating targets I use is to use the shrinkwrap modifier to fit the mesh to another object, then apply as shape key. But as far as I know, shape keys created outside the addon aren't recognized by maketarget, and so I get around that by making a copy of the human mesh with the shapekeys applied, then using "load target from mesh" to get it into maketarget.
(To load target from mesh, select the mesh you want to use as a target, then shift select the mesh you want to create a target on. This consumes the first mesh and creates a target on the second mesh.)
Also, when creating any target that affects the body, apparently you have to use the helpers/fit tools, otherwise the clothes won't fit later on.