If you just want the mesh as one contiguous object without separations at the body part groups, generally most contemporary Poser figures have such a mesh, as referenced by the cr2, contained in the Geometries folder of the figure in question Runtime. It will be in zero pose however.

There's also another option via export directly from Poser scene, posed or zero, that maintains groups (optionally), material regions, and UVs but will change the vertex winding order. The winding order is expected to match as initially created for original morph targets to work properly (the ramifications and solutions to this are a more involved topic)... If pre existing morphs are not important then...

For this demo I'll be using a posed La Femme Poser figure at base resolution

To start

File: Export: Wavefront OBJ

Select none in the pop up scene Hierarchy and then check the topmost indicator for the figure. This will select all of the actors that comprise the figure and nothing else...

Use these settings in the export dialogue if you wish to maintain body part groups and materials...

Export to a location of your choice and give the model a unique name.

Next comes the welding bit. You might have noticed that there is a Weld body part seams option. I prefer the results of an import weld.

File: Import: Wavefront OBJ and select your exported model

Check this option, Weld identical vertices, for the import

You may want to hide or delete the original figure. You now have a welded prop of the figure in scene... polygon groups (if you have chosen), material regions and names, and UVs are intact...

Here are the welded props groups and materials demonstrated

Now export your prop again overwriting your initial export so that you can have the welded model external for further use in other software. Again if you want mats and groups maintained overwrite with these options checked...

Here is the welded model in Mudbox, a sculpting software...

Here are it's UVs, UDIM in this case...

The model is suitable for 3d painting (resulting texture painting files suitable for the original figure) and sculpting, here's an aggressive smooth applied to the arms to show that there are no weld splits...

You can also create more elaborate figure props which include clothing and hair models merged with the figure.