How to crate a list using information from another list

Hi,

I am reading data from Excel and I am trying to create strings using data from a list (which is read from Excel) to create a new list. For example, for rows 1 to 5 in Excel I have:
1
2
3
4
5
I need to create:
1
2,3,4
5
I tried a Python script to create the string using ‘+’ but my output was:
1
2
,
3
,
4
5
Does anybody know how to do this?

Like this?

Thank you Smaren, Your solution definitely works for this case but the problem is that I need a method to be applicable to a large scale problem with different numbers of sheets and list lengths.
Just to give you an idea if you can check the attached Excel file, Dynamo reads Circuit numbers of each PANEL and if the number of Poles is equal to 3 then I will create a string with the next two circuit numbers. For example for the left side of panel A I have:
1
3,5,7
9
11
Sample-Excel.xlsx (211.2 KB)

I used .append instead of .extend and it solved my issue.