There are a couple of options.
One is to have your original model selected and use Tool >> Morph Target >> Store MT. You would then (with the model still selected) import the morph version. That version will replace the original geometry. You can then use the other options in the Morph Target sub-palette to blend between the two versions as you wish.
Alternatively, you would use Tool >> Layer to create a new layer before importing your morph version into the same model. You can then blend between the versions using that layer’s slider.
Both of these options require both versions of the model to have identical points, which you say yours has. If your morph target didn’t have identical points you would need to use the Layer method and instead of importing the mesh into itself you’d have to append as a SubTool and then use Project All. That doesn’t apply in your case, but I’m supplying that info so that you’re aware of it for situations when the points are different.