Not sure what you are saying but you need to keep the flange. A new post need to have the same diameter as the standard post.
It is also possible to use a telescopic solution. Standard size post enters the bike but at the top of that post another post is inserted. This is a heavyer solution but also I find a telescopic seatpost to be a good solution for travelling.
Make sure you need extra suspension before you buy- and make sure it`ll actually work for you. You are wery light (weight) so a suspension that works for a heavyer person may feel rock hard for somebody your weight.
Maybe just get a softer suspension block for your bike?