When you are adding the new vertices and polygons, be sure to group them with the same GROUPS as those around them (how to do this will depend on what modelling software you use - but will be a feature called groups or parts most likely).  In many versions of Poser since the Poser 4 Pro Pack you can inherit a skeleton from an already rigged figure with a new figure, provided the groups have the same internal names (this is critical).  There are tutorials both here in Renderosity's Tutorial archive, and over at Daz3d for doing this.

Also, usually to modify meshes you do not have to "detach" the skeleton, you don't usually even have to export them back out of Poser.  In your modelling software, when importing the object, go straight to the geometries folder in Poser and you will usually find the geometry right there, usually already in Wavefron OBJ format.  With some props and a few figures they may have embedded geometry and require you to export, but I always check the Geometry folder first (just be sure to save it with a different name when you are done, or to a different place - or you will lose the orriginal).

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