The net resistance of any 2 resistors in parallel is
xy/(x+y)=y/(1+(y/x))
Let's say you fix y, then there is no way you can get a linear variable resistance by adjusting x. If linearity is not important, just go ahead and let the net resistance be 50k, max(x)=100k and find the corresponding y.
Or alternatively, maybe you can show us your schematics, and we'll see if anything else can be done to get the same effect. But of course, the easiest method is to just replace that potentiometer.