Quote - I guess "supernatural" is an wrong term. Simply I want to make a man in Poser with a frog's head and a man's body.

 

Poser has lot of limitations when it comes to this.
For example, in Max, you may be used to stretching and moving and welding polygons, or filling in the feometry to join the body and the head, when they came from different figures.
In Poser, you can't do that (well, you can, but in a rather frustrating and roundabout way if you're used to doing things in Max.)
In Poser you would have to look for a cpecific figure, for example, Michael3 made by DAZ, and then look for add-ons made specifically for that figure, so you don't have to mess with joining parts that don't join on their own.

Now, if you want to take advantage of Poser content available in many places, in Vue and Max, you really don't NEED Poser at all. You could use DAZ Studio (free at DAZ), and collada exporter, and few native exports from DAZ studio, and you'd be in business. 

Unless you're in love with Poser or want to resell Poser content, and since you're used to Modelling in Max, you're much better off rigging things in Max.

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