With GraphPad Prism, is there a way to force the slope to equal 1.0 in a linear regression?

Not with "linear regression". But choose nonlinear regression, and choose to fit to a straight line model ("polynomial, first order"). With nonlinear regrsesion, you can constrain the intercept or slope to whatever you want. There is no problem using a nonlinear regression analysis to do linear regression. You'll get the right answers.