Using normal economics mode you can't do that since you simply enter the assumed nominal rate of return. However, in Monte Carlo mode, (if you have PLUS) you can enter specific asset positions for each year so you could create the glide path that way. Just enter the particular asset positions you want in whatever proportion you want for each year.

With respect to changing asset allocations each year, if sdrake doesn't have ESP+ already s/he might not know you're limited to 10 different portfolios. If the intent is to reduce it 1% point each year over the next 20 years, you can't do that.

That level of detail may not be necessary. A change of 2% points every 2 years may serve the purpose, but there is a limit of 10 separate portfolios.