I have an old Kodak DX7590 and I was wondering if there were any lens adapters that would work with it to turn it into a mediocre DSLR
No. No lens adapter or add-ons can turn a a fixed-lens, small-sensored (1/2.5" format) camera into one with an interchangeable lens mount and large (APS-C) sensor.
However, the DX7590 can use a filter adapter tube so you can put a 52mm-diameter filter in front of the camera's lens. The type of filter you choose can extend function. You can get teleconverters to increase or decrease the focal length, you can get close-up filters to enhance macro capability, or simply use regular filters like infrared, neutral-density, or circular polarizers. But your image quality with these filters will either remain roughly the same, or be reduced (particularly in the case of teleconverters). That's just how filters work.
Well, there is a .45X adapter available under a number of brand names (including the original Kodak accessory in the used market) that will take it from a 38-380mm equivalent to a 17-170mm equivalent for about $50-60 new, or less used. The optics are "good enough"; the camera is a 2004-vintage 5MP machine with a 1/2.7" sensor, so you can't expect miracles. It will vignette (have dark corners) when you're zoomed out all the way, so its effecctive zoom range will be more like 21-170mm (35mm equivalent).
Nothing, though, is going to allow you to get things like shallow depth of field (except in macro/close-up mode); that requires a larger sensor and the longer lenses it takes to give the same field of view on the larger sensor. One way to get something more closely resembling a mediocre DSLR is to buy a used entry-level (even mediocre) DSLR. You won't have anything like the zoom range (range of focal lengths) you're used to, but something like an 8MP Canon Digital Rebel/300D or a 6MP Nikon D40 or D50 can often be picked up in barely-used condition for under a hundred bucks (slightly more with an 18-55mm kit lens) if you look around a bit.