You can't unfortunately. You can only make it lower.

Witness the math for resistors in parallel: R(x) = 1/(1/R1 + 1/R2 + ... + 1/Rx)

In the case of two resistors, it can be stated this way:

1/Rx = (R1+R2)/R1*R2

Say R1 = 50 and R2 = 100

1/Rx = 150/5000 = .03, therefore Rx = 33.3k

In fact, as R2 gets exponentially large, you will only approach the value of R1 by fractional amounts. You would have to have an infinitely large resistor just to get back to your original 50k (only in pure mathematical terms, not practical ones)!

So, you cannot increase the value of a pot with a parallel resistor, but on the other hand, you can reduce the value of a pot that way which sometimes comes in very handy.