Combining 21-day pass with Kansai Area Pass

Hi!

We will stay in Japan for 24 days and would like to combine this with a Japan Rail Pass (21 days) for the first 3 weeks, followed by a Kansai Area Pass for the last 3 days.

My questions:
1 - Is this possible / allowed? I know you you can't combine the regular rail pass, e.g. 2 times 14 days, but I did not find any information about combining a JR Rail Pass and an Area Pass.
2 - If allowed, is it possible to order them together through this website? I tried, but every time I tied adding either of them, it overwrites the other one in the order.

Kind regards