Although I refer to the Canon G7, I have also carried out this modification on a Canon G9 housing (WP-DC21).
The Canon housing for the G7 & G9, like most other Canon housing doesn't have a filter thread on the lens port.
With previous Canon housings, Inon made a mount base so that lenses could be attached.
I checked with Inon to see if they have any plans to make a mount base for the Canon G7 housing (WP-DC11) and the answer was no.
I think that some 3rd party mount bases and 67mm lens adapters are now available.
The standard Canon G7 housing (WP-DC11) looks like this.
Unlike the Olympus, Canon don't design their housings with filter threads. On the G7 housing, Canon didn't even make the lens port round.
Of course you could always purchase the more expensive but better built Ikelite housing, which comes with a filter thread.
I decided to see if I could fit a filter thread to the Canon housing, so purchased a stepping ring and removed the rear thread, then drilled four holes in it to line up with the existing screws in the lens port and fixed it there using the same screws.
After modification, the housing now looks like this.
If you are interested, here are the details of how I did it.
1) Obtain a 62mm to 67mm stepping ring.
2) Unless you know anyone with a lathe, remove the 62mm thread from the back of the ring by carefully cutting it off with a small hacksaw.
3) File down any large burrs
4) Place the ring on some fine grade Wet & Dry paper, and gently move it around until the back is completely smooth.
5) Find some clear cellophane sheet (or something see through) large enough ( 7cm x 9cm minimum size ) to cover the lens port on the housing.
I used cellophane from the packaging
6) Hold the cellophane over the lens port and using a sharp pin or needle, mark the centres of the two screws at the top and the two screws at the bottom, by pushing the pin through the cellophane.
After you have done this you will have a template with four holes in it. You may find it easier to see the holes needed for the next step, if you draw round them with a felt tip pen.
7) Cut two pieces of sellotape, about 2cm longer than the sides of your template, place them sticky side up on a flat surface, so that they form a cross, with one on top of the other.
Then place the stepping ring, smooth side down, in the centre of the cross, so that it sticks to the sellotape.
Next hold the template over the ring and carefully position it so that the holes you made in the template are now above and centred in the middle of the ring's rim.
Then fold the excess sellotape over the template locking it in place.
8) Remove the fours screws (two screws at the top and the two screws at the bottom) from the lens port and find a drill just a fraction bigger than the width of the screw including the thread.
9) Find a sharp metal spike to use as a centre punch and push it through the holes in the template to mark the centre of the holes to be drilled.
10) Remove the template from the ring. From the thread side of the ring, carefully drill the four holes through it.
If you nick the thread slightly when drilling, it doesn't matter too much provided that you clear the thread of any swarf of burrs afterwards.
You may want to get someone to help you hold the ring still on a piece of scrap wood, while you concentrate on the drilling.
11) Find another drill just fraction larger than the head of the screws. Use this drill to slightly countersink the holes you have made in the ring.
So that when the screws are in place, the head is level with the surface of the rim, otherwise you will not be able to fully screw your lens onto the ring.
12) Using some matt black spray paint, spray the back of the ring where the thread was removed and dab some paint around the holes in the front.
13) When the paint has dried, attach the ring to the lens port using the the four screws you removed earlier. Tighten the screws firmly to the same level of torque you used to remove the screws.
Hopefully if anyone from Canon or Inon reads this, they will realise what little effort is required to either design in a filter thread or sell an after market kit to do this.
While we wait for Canon or Inon to do something
Please feel free to copy my example, but if you do, please remember it's at your own risk, I take no responsibility for design or workmanship.
I used another stepping ring for a makeshift lens caddy and piece of crimped stainless steel for a reflector. Lately the reflector is redundant as I find it better to move the diffuser to the end of the macro lenses instead. If you are using an external strobe then you will have no use for either a reflector or a diffuser on the housing.