I am assuming your pants are conforming clothing?
There are several ways to do what you want, but you can try this:

1. Export from Poser your pants as a Wavefront obj. Select the Save as Morph Target option. Make sure you ONLY export the pants.
2. Import into your 3d modeler of choice.
   Note: The file will import in a very small scale. If that is an issue, you can use objscaler, just remember that if you scale the file, de-scale the files intended for Poser.
   If your pants are conforming clothing, you will see that you have different mesh groups corresponding to different body parts in Poser. So this can be a bit of a problem when doing morphs (but that's how it works)
3. Do your morphs. Don't weld vertices, don't re-group meshes.
4. Export each body part that you morphed as a single file (that is, one file for hip, one for lbuttock, and so on)
5. In Poser, select each body part, and then with the menu, Object->Load Morph Target...
   Choose your file corresponding to that body part. Rename the morph to whatever name you want
6. Do no. 5 for all body parts in the pants that were morphed.

If everything went as planned, you'll have new morph dials in your pants!
If you want to make a full body morph of the pants in the morphed state (so you can select the body part, and just spin a dial that will morph the whole pants in one go), set the morphed state of all the pants parts as you want them, and then do Figure->Create Full Body Morph.

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