You can, but I would not say it will be trivial to do. The skin as designed displays current, today/tonight and tomorrow.
If you want more than 3 days returned to the skin, you will need to change the Variable @Include3 in the skin, to one of the other .inc files that return more than the 3 days I have set by default. Instructions are in the skin.
Then you will need to add more "meters" to display the extra days. You can use what is there now as a template for how they should be coded, being careful to remember that every [section] in a Rainmeter skin must have a unique name.
Then change the meter [MeterAllBackground] to be larger/longer, to hold the new meters you create.
If you are willing to dig into coding in Rainmeter, adding more days is not terribly difficult. However, be aware that it's not really easy either.