daroc,

how are the data in file B organized? In consecutive rows? If so, you
can fill column B:B in file A with the following formula:

=IF(MOD(ROW(),2)=0,OFFSET('[fileB]Sheet1'!$A$1, ROW()/2,0),"")

Then you fill column C:C with the following formula:
=IF(A1<"",A1,B1)

And copy down as far as necessary.

HTH
Kostis Vezerides