Say your cities are in column A. In a helper column, enter =A1 & "," and
copy down. Then copy the helper column and paste/special value back into
your original column.
I'm assuming you want a formula for this.
With the city in A1:
=A1 & ", "
The formula above also includes a space after the comma with assumption that
you will be adding the state. If your state is in B1, then the formula
would be as follows.
=A1 & ", " & B1
And with the zip code in C1:
=A1 & ", " & B1 & " " & C1
Say your cities are in column A. In a helper column, enter =A1 & "," and
copy down. Then copy the helper column and paste/special value back into
your original column.