has anyone any information on adding a donut spare tire for my g37 convertible? One dealer says it is not possible and another says it cost $600 jus for the wheel.

 

You may want to check this article from infinitig37.com regarding the physical spare tire and the convertibles. Apparently there was a kit that you could order to have a real spare tire instead of these (useless (IMO)) "Tire Repair Kits."

A brief check shows these kits as "discontinued" though I am sure with some web searching you might find one kicking around. Also, eBay may be a option, if you would consider used items. You will likely have to individually source the components needed.

The bracket that you have to install on the trunk floor (referenced in the above article/kit) is available from parts.infinitiusa.com and costs approx. $14 (as of 4 Nov 2021) not including S/H.

 

Mine came with the spare. Not the full size. Wouldn't help any ways as front and back wheels are staggered.

 

Whilst I normally would not reply to dead threads... to correct misinformation I must.
Your "donut" spare will work whether your tires are staggered or not. Mine works just fine. You just have to drive "conservatively" until you get your tire repaired, but it will keep you moving and it sure beats standing on the side of the road fiddling with a repair kit or waiting for AAA.
No car comes with "full sized" spare tires any more. Hell, most newer cars don't come with ANY spare tire at all just a absolutely useless "repair kit."