I have 2 walnut slabs I want to make a table with but they are slightly warped, is there anyway to flatten these back out with water and weight or by bolting the wood down to a frame and then moistening? Not sure and don't want to make the warping worst than it already is?
We have to be careful in advising you here since what you mean by "warp" may not match the commonly accepted definition. Some people use the term generically to refer to boards that aren't flat along their length, rather than to describe a specific kind of curvature to which the word more accurately refers.
You can sometimes improve on this by dampening one or both sides, but there are no guarantees that it'll work as a lot depends on the specifics: when and how/why it occurred, what kind of warping and its severity and last but not least the wood itself, both species and how it's cut (e.g. flat-sawn/plain-sawn v. quarter-sawn). As well as this the individual nature of the piece (how the grain runs in every direction) can be a big factor.
So before anything else do be aware that some boards inherently want to warp in some way because of their grain (see image here) and that's the first thing to look at to assess the situation.
When you probably shouldn't bother trying
If the wood was in this shape when you bought it there's a decent chance that it took on that shape naturally, to relieve internal stresses in the board during drying.
A flat section from within such a board (created by planing off the low and high spots) is usually fairly stable and will often/usually remain flat. This is how wood that is slightly non-flat, as much rough wood is from the sawmill, is commonly flattened off in woodworking to prepare it for use.
If on the other hand such a board can be made flat again using some combination of water or steam, sunlight, weight or clamping pressure it is very likely in line for re-warping.
When it has a chance of working
The prime candidates here are boards that have cupped or warping since they were bought.
There are two situations I can think of where this commonly occurs. The first it if the wood is stored flat on a surface with the top exposed which doesn't seem like a bad thing, but it can lead to differential moisture exchange between the top and bottom surfaces (if the board is drying the bottom can only do so much more slowly, and the same in reverse if the wood is taking on moisture due to high humidity).
Another reason is if the boards got damp on one side (even a light sprinkling of rain onto the boards as they're carried into the shop from the car can be enough) and then were not dried out stickered while being held flat with weight.
If your slabs might be curved from this kind of irregular drying trying a fix is certainly worth trying IMO as you're very unlikely to make matters worse. Some improvement is usually possible, and once or twice I've gotten boards back to completely flat. I then stored the wood under clamp pressure to keep it that way until just before it was used. Wrapping the wood very well in a plastic bag is another way to keep it stable by preventing it from taking on or losing moisture.
Related previous Questions with some more details:
How do you straighten/bend wood planks without removing any material?
How do I correct extreme crossgrain cupping?
Essentially: No. You make slabs flat by removing wood to obtain a flat surface, by planing or by using a carriage-guided router or by machine-sanding or some combination of these. Sometimes you can reduce how much wood is lost by dividing into smaller pieces and gluing back together later, but making the grain match up again is a challenge.
See the related questions here about warped wood.