Save the character to the library (let's call it CHARACTER 1.CR2.. When you save it, it will save an OBJ file with the same name (CHARACTER 1.OBJ) into the same folder.

In the meantime, take your ORIGINAL OBJ file back into your modeler. Fix the opened polygon in your modeler, and resave the OBJ file. For this example, I'm calling it CHARACTER 2.OBJ

In a blank scene ...

1) Add the CHARACTER 2.OBJ with the fixed polygon into the scene.

2) Open the Poser library, and navigate to the folder where you saved your original CHARACTER 1.CR2 file

3) Select the CHARACTER 1 library item, and then click the single checkmark at the bottom of the library window. Poser will ask you if you want to replace the current figure's skeleton. Answer OK.

4) In the Transfer Figure dialog that appears, make sure you check Auto Group to add any groups that you set up in the original version. You didn't mention if you had morphs. If you don't you can uncheck "Transfer Morphs."

5) Then click OK. 

When you return to the Pose room you should have a new poseable character named CHARACTER 2 that uses the new fixed geometry. Save that new version to the library as CHARACTER 2.CR2. Delete the CHARACTER 1.CR2 if you don't need it anymore. That should do it.