It isn't a formula, but this could work:
Highlight the numbers you want.
Copy using CTRL+C
Create a new word document
Paste Special using the edit menu and paste it as plain text.
Press CTRL+H and in the find field put ^w and in the replace field put ;
Still on the replace menu, put ^p in the find field and ; in the replace
field.
Visually inspect the data to make sure there are no ;;; (There were in my
test data because I had blank rows). If there are, find ;;; and replace with
;.
Now copy that text, and paste it back into Excel.