Where did the water come from?
The Fremen were collecting water, under the guidance of Dr <PERSON> from the atmosphere into secret catch-basins using hidden wind traps.
Station tools began finding their way into the sietch warrens - especially cutterays which were used to dig underground catchbasins and hidden windtraps.
Water began collecting in the basins.
Dune - Appendix I - The Ecosystem of Dune
And from various plants, cactus like in their retention of water:
There was a rare native root plant that grew above the 2,500 meter level in the northern temperate zone. A tuber two meters long yielded half a litre of water.
Dune - Appendix I - The Ecosystem of Dune
The water extracted from the deathstills† would also be available to be added to the catch basins.
When it was done, <PERSON> clapped his hands once. Attendants came and removed the bodies, taking them to the deathstill where they could be rendered for their water.
<PERSON>
Sandtrout were also placed into deathstills to extract their water.
And She thought Sandtrout? Many times in this flesh and other had she played the childhood game, poling for sandtrout, teasing them into a thin glove membrane before taking them to the deathstill for their water.
<PERSON>
This method as well as getting water will also remove some of the Sandtrout that are part of the Spice>Sandtrout>Sandworm cycle. Though this cycle would not be able to be broken until the terraforming of Arrakis begins with vast quantities of water
Using all of these methods <PERSON> estimated that it would take:
In the manner of a teacher answering a child who has asked the sum of 2 plus 2, <PERSON> told them: "From three hundred to five hundred years."
Dune - Appendix I - The Ecosystem of Dune
Following the establishment of a proto-ecosystem of various grasses and a few hardier plants in some protected areas of Arrakis the estimate was locked down a little further:
From the charts emerged a figure. <PERSON> reported it. Three per cent. If they could get three per cent of the green plant element on Arrakis involved in forming carbon compounds, they'd have their self-sustaining cycle.
"But how long?" the Fremen demanded.
"Oh, that: about three hundred and fifty years."
Dune - Appendix I - The Ecosystem of Dune
Then came <PERSON>, who upon becoming emperor increased the pace of water collection to create the terraforming needed for the vision of the golden path he had, including the use of weather satellites to bring rain. 321
This was taken up by <PERSON> after his father could not continue on the golden path, until only a small personal desert, the Sareer, was left in <PERSON>.
As for food.
There is the Muad'dib mouse that <PERSON> took his Fremen name from, this could be eaten, as could anything that preys on it.
There are also carrion birds mentioned by <PERSON>
There are carrion eater birds over me. Perhaps some of my Fremen will see them and come to investigate.
Dune
These again could be eaten, and presupposes there is an active enough ecosystem on Arrakis to support them.
Plants are shown to be tended in the sietches using plastic sills to gather water and funnel them to the roots.
One could also guess that mushrooms would grow in the various caves that make up a sietch.
Following the establishment of the nascent ecosystem the following fauna and flora were introduced:
They turned then to the necessary animal life - burrowing creatures to open the soil and aerate it: kit fox, kangaroo mouse, desert hare, sand terrapin . . . and the predators to keep them in check: desert hawk, dwarf owl, eagle and desert owl; and insects to fill the niches these couldn't reach: scorpion, centipede, trapdoor spider, the biting wasp and the wormfly . . . and the desert bat to keep watch on these.
Now came the crucial test: date palms, cotton, melons, coffee, medicinals - more than 200 selected food plant types to test and adapt
Dune - Appendix I - The Ecosystem of Dune
All of which would conceivably be used to supplement their existing diet.
†a machine used to extract water from a body, usually dead, though the person can be alive when used as Fremen capital punishment 779