You could do that, but it might be just easier to change the DEST in the FLT PLN page once you're heading towards your alternate airport. The go back into the LEGS page and build the route to the alternate. First, get the airplane going to the first fix on the route. Let's say that Wichita KS (KICT) is your destination and Hutchison KS (KHUT) is your alternate. You're tired of shooting and missing the ILS 01L approach at KICT due to fog, etc., whatever. Time to go to KHUT. Wichita gives you the following clearance to KHUT: "N1234 is now to cleared to the Hutchison airport via direct Wichita VORTAC, Victor 73, Hutchison VORTAC, then direct, climb and maintain 6000". By the way, this was a common clearance we gave in the sim when I was an instructor in the Learjets while at FlightSafey.
Step 1: On the LEGS page, type in ICT to go direct, then EXEC
Step 2: On the FLT PLN page, change DEST to KHUT
Step 3: On the FLT PLN page, delete any remaining approach going into KICT
Step 4: on the FLT PLN page, type in:
V73 HUT
DIRECT KHUT (this is optional)
Step 5: On the DEP/ARR page, select KHUT and enter the expected approach, e.g., ILS 13
Your performance data should remain, i.e., PAX and CARGO, ZFW entry, etc.
With ALT LEGS page, you can set some of this up ahead of time. However, as practical matter, I have never used the ALT LEGS page to preload my flight the alternate. Most of the time if you are diverting to your alternate, ATC will clear you direct. In mountainous areas, that might different.
ALT LEGS page is mostly used for programming your engine failure departure procedure, so that it is ready to use if the engine fails on takeoff after V1, assuming that it is not a straight-out departure.
Rich