just add data to where the data is looked up from, then check that the formulae that refer to it still got that far in the area that it is looking.
i.e. if the existing formula looks something like
=VLOOKUP(A1,Sheet2!$A$1:$B$20,2,FALSE)
then the cell is going to look for the value in A1 in sheet two, looking in column 1 and returning the value in column 2, in the range A1 to B20
If you added extra data and the last row you want to now look in is 300 then you would change $B$20 to $B$300
or start using named ranges replace all of $B$20 with a named range.