appending to a line that doesn't match grep

I have a file which contains:
ss1 dest1 dest2 dest3
ss2 dest2 dest4


I get the dest2 for ss1 say from reading another file.
Then I will not append dest2 to ss1

Output file:
ss1 dest1 dest2 dest3
ss2 dest2 dest4

If I get the dest4 for ss1 say

Output file:
ss1 dest1 dest2 dest3 dest4
ss2 dest2 dest4

How to achieve this? Please suggest.