No. If you want a digital SLR then the only way is to buy one. There's no quick cheap option to just fit a digital back to your current camera.
Used digital SLR's may be cheaper, but I recommend avoiding them: they're really complex pieces of equipment with lots to go wrong and few last longer than five years anyway. Owners with intermittently faulty examples will often just sell them to an unsuspecting buyer so they're always a huge gamble.
If you have a few Pentax lenses and would like to continue to use them, then Pentax digital SLR's still accept them but the smaller image sensor compared to 35mm means that your lenses will produce images which appear to be magnified by about 1.5x. You can save a lot of money by buying the near identical Samsung branded versions, or save a bit more and buy a "bridge camera" which has an electronic viewfinder and a superzoom lens permanently attached. Image quality is not as good, there's a limited aperture range, but they are more compact and convenient than carrying around a bulky SLR and a selection of lenses. Some, like the Olympus EP-1 and Panasonic GF-1 have interchangeable lenses and are halfway between an SLR and a bridge camera but they are definitely not cheap.
There is one other way to go digital and it's to do it the way I did: I still far prefer 35mm for it's reliability and build quality, plus the fact that equipment is often free or dirt cheap. Although I shoot film, I develop the negatives at home or take them to the local minilab where they do so for just £1 a roll. I then scan them using my flatbed scanner which has an excellent film scanner built in (
Epson Perfection 4870 Photo, £48 second hand). I can then store the negatives safely away for archival purposes and have digital images to work with which are superior to any digital camera which I've had the chance to use. My only real expense is that I've had to invest £80 recently for a 1TB hard drive because I always scan my negs at the highest possible resolution.
Using 35mm gear means that I have no battery worries and can continue shooting in conditions that nobody else will risk their digital SLR in.
Just try to remember that the object is to take photos, it doesn't
really matter what you use and it's really easy to make the mistake of
thinking that "better" equipment will make you a better photographer.
Usually it just means that you spend so long fiddling with all the settings
and deciding that the weather is too risky for your expensive toy that
you miss brilliant opportunities for great shots.
I hope that my reply has been of some assistance, if so please rate my answer.
