I have not found that lens on the Olympus housing port charts, which leads me to believe that there is none from Olympus.
Olympus makes a nice dome port for 8mm and 7-14mm.
The 20mm "pancake" lens does not stick out very far, so I expect it will be useless with the Olympus port or any of the Zen ports (you will be taking pictures of the insides of the port body.)
The PT-EP03 housing with standard port is compatible with (according to Olympus website)
- M.ZUIKO DIGITAL 14-42mm 1:3.5-5.6 II
- M.ZUIKO DIGITAL ED 14-42mm 1:3.5-5.6
- M.ZUIKO DIGITAL ED 9-18mm 1:4.0-5.6.
I suggest you look into the cost of getting the 9-18 lens plus zoom gear and compare that to the cost of finding a port to work with your 20mm pancake lens (if a compatible port exists). The 9-18mm lens will not be as wide when used with the standard flat port, compared to one of the dome ports, but it would be less expensive way to get into wide angles.
The Olympus 9-18mm lens is very nice and this week there is an Olympus "Instant Rebate" on it, so it's only $600, then add the zoom gear for $90. So for $690 you would have a nice lens and zoom gear. You could then go to one of the dome ports sometime in the future when you want to get the widest angle performance out of the lens.