outdoors coming in? help a lady out before the frost

this spring I planted 7 sick plants in my front garden. they took off, and soon grew into huge bushes. late july it had to dig them up, and moved them inside. they are doing so good and show no imbalances or defiencies even late in flower(2 sativas auto flowered in june). I just give them phed water, and nutes once a month. the only problem I find is that the soil doesn't hold moisture as well as i would like.

I've been growing in/outdoors in pots with coco and due to water issues have beenhaving nothing but problems.

the soilhere isgreat and grows monster plants.
1 I want to know if I should use the garden soil
2 will mixing in a little coco to hold water affect nutrient accessability of the soil, how much
3 the frost will be here soon. how shold I prepare the soil this fall for next year? I was thinking of tilling a bunch of coco into the garden, or into a pile.
4 I need to know more about large outdoor composts, we moved here and there is one here and we'veadded lot to it, I just need a place to catch up on reading.
thanks
ps keyboard is broken again, response may take time