I'm going to start by assuming that the original groups are preserved in your new clothing. Best to check this first. If they are not then let us know.

Import your OBJ file with no import option boxes selected.
Load V3
Turn off all V3 IK chains (Figure -- IK)
Open the joint editor (Window -- Joint Editor)
Click the zero figure button.
Select the V3 hip
Check that all translations are zero. (My copy is, yours may not be).
Repeat for the BODY.

At this point the clothing MUST fit the figure. If it does not then you will have to edit the OBJ so that it does.

Select the clothing so that it is the active actor in the scene.
Click on the Setup room tab
Open the library window and browse the figure library for V3 BLANK.
Double click on the V3 BLANK thumbnail. This will add the V3 bones to your clothing.
Return to the Pose Room.

Ensure that your new clothing figure is selected.
Turn off all V3 IK chains (Figure -- IK)
Open the joint editor (Window -- Joint Editor)
Click the zero figure button.
Select the clothing hip
Check that all translations are zero. (My copy is, yours may not be).
Repeat for the BODY.

Now -- very important click on  Edit -- Memorize -- Figure

Save your new clothing to the figure library.
Test your clothing by conforming it to V3 (Figure -- Conform to)
Adjust joint parameters if required. This is normally limited to increasing the fall off zones to some parameters if you find that some of the cloth is being left behind when a joint is rotated. Do not move the joint centers.

Hope that helps.