You would need to force the air coming out of the leaf blower into a really narrow tube (to get PSI up), and attach that tube to a tank (to ensure constant air discharge), and then run an air hose (with PSI gauge) to your gun. you probably could only get the PSI up to the 60+ you need to run a gun by sacrificing your volume (CFM would basically be at zero). The only way it could work is if you fill the tank over a long period of time, and then discharge it at the CFM/PSI you want (but you would have to use most of your leaf-blower runtime on filling the tank). You would need a really big tank. two or three propane tanks would give you enough air to run your gun for a fair amount each time.
Honestly, It could be done, but just the labor required to make an airtight connection to the compression pipe/the cost of the parts you would need justify the Standard Eaton. (eatoncompressors.com). I've had really good experiences with their products. They're failrly priced, and they run really well.