Hello there.

If player A has permission A for 10 Days or sits in group A for 10 days, is it possible to yet again give the user for example 30 days on his permission or group access? and, if you give the user extra days, would it count up to 40 days or would it change to 30 days instead? 

Looking at the plugin's algorithm, it works by adding more to that period if the player already has access to that group for a certain period of time via Timed Permission. So the answer to the question is a total of 40 days.