I've sprouted cherimoyas before - I believe I nicked the shell first (it's been ages). Unfortunately I can't help with climate because I had to sell them (was moving!)... but here's some info. that might help:

http://www.cloudforest.com/community/topics/2.html

good luck!!

Oops - I had 2 pgs. open at once and put in the wrong link :) that other one wasn't very informative... this might be better:

http://www.naturalhub.com/grow_fruit_and_nuts_in_tropical_areas.htm

CHERIMOYA Annona cherimola-Medium to very large most sweet and complexly flavored fruit with soft white or cream flesh with numerous bean sized shiny black seeds embedded in it.The fruit is carried on a small tree that is amenable to pruning. It can also be informally espaliered. It regrows easily from a severe pruning-handy because the wood is fairly brittle and liable to hurricane damage. Cherimoyas need a period of cool temperatures (although they are damaged by actual frost), and are therefore only successful in cooler elevated areas of the tropics. They require very little care beyond pruning after fruiting, and intermittant fertilising with a complete fertiliser. They need fairly good drainage or they will get root rot. A thick organic mulch helps in marginal soils. A grafted tree should start fruiting within 2 or 3 years of planting out. Any grafted tree will have lovely fruit. Some cultivars have smoother flesh than others, or have a slightly resinous taste, or the flesh is whiter-but the difference is between 'delightful' and 'fantastic', so it doesn't matter. Cherimoyas are picked while still firm-usually when the green skin takes on a very slight yellowish tinge. They will be ripen in the fruit bowl about 4 days from picking.