There are two 24/85 lenses that NIKON make, one has a maximum aperture of 3.5 to 4.5, but from your description of a "macro setting" I am assuming you have the 24/85 2.8-4 AFS lens.
This was the first lens that I bought when I got into underwater photography with a land camera and housing (NIKON F90X) because it looked like a good "allrounder" a nice bit of "wide"(24), a bit "long"(85), and some "macro" thrown in. So I looked at the SEA and SEA port chart for a good "allrounder" port as well,
http://marine-cameras.com/lenses/ports.htm
and I came up with the SEA and SEA NX Zoom port (PART NUMBER 51101, part number 3 on the chart) which seemed to cover a lot of the lenses featured in the port chart, unfortunatly it was just not big enough to allow the 24/85 to extend its full lenght, so I also purchased the SX extention ring (50261, part 13 on the chart) to allow for this, now this set up does not give me full use of the lens because at the 24mm end the edges of the port are in veiw, but it does give you the fullest use of your lens under water.
With the "macro" lock on, your lens becomes a 35/85mm lens with "macro" facility which will cover a lot of photographic situations underwater. Just don't forget to lock the lens into "macro" mode befor sealing the housing.
Nobody makes a zoom gear for this lens, but if you look at my post in the D.I.Y. Section of wetpixel, you will see my solution.
http://wetpixel.com/forums/index.php?showtopic=15291
This port set up will cover both 24/85 (Zoom port + extention port)
AND a 60mm MACRO lens (Zoom port, no extention port)
I dont think that you will get good results from the 24/85 behind a dome port because the lenght of the lens changes so much during zooming.
Don't worry about Teleconverters for now, read up on other websites about what they do, or get a book about close up photography 
Start with your YS 90 strobe, it should be OK for macro work, you will have to adjust the light levels manually as I am sure you are aware when you used it with your P a S camera, Position your strobe directly above your lens, looking down at the subject at approx 45 degrees, any shadows will be behind the subject then.
Hope this helps.