[vim] How to add text at the end of each line in Vim?

There is in fact a way to do this using Visual block mode. Simply pressing $A in Visual block mode appends to the end of all lines in the selection. The appended text will appear on all lines as soon as you press Esc.

So this is a possible solution:

vip<C-V>$A,<Esc>

That is, in Normal mode, Visual select a paragraph vip, switch to Visual block mode CTRLV, append to all lines $A a comma ,, then press Esc to confirm.

The documentation is at :h v_b_A. There is even an illustration of how it works in the examples section: :h v_b_A_example.