You didn't say what version of Poser you have. That's always important to note when asking how to do something specific.

If you have one of the pro versions, then you can combine multiple morph dials into one dial by going to the Figure menu and selecting "Spawn Full Body Morph". This will create a new FBM based on every morph you have active at the time. Active means anything with a value greater than zero. The dial will show up on the Parameters pallet under morphs.

If you have PP2014 or P11 Pro you can also use GoZ from Poser to ZBrush, sculpt your morph on the figure, and then send it back to Poser as an FBM. It has to be done all in one session though. If you close Poser or ZBrush at any point in the process then any work you've done in zbrush will be lost.