Both files A and B must be opened;in source file B, place the cursor in cell F29, go to the Insert menu, then choose Name, and then Define from the expanded menu . Enter a name, like F29FileB (no spaces), ( in refers to box you will already have =Sheet1!$F$29 (assuming that the source cell F29 is in sheet1). press enter. We are done with source file.
Now, in destination file A, define another name , could be the same from source: F29FileB, clear everything there is in the refers to box, then switch windows to source file, click on cell F29; now, in refers to box, you will have
='[File B.xls]Sheet1'!$F$29 which must be edited to the name defined in source: ='File B.xls'!F29FileB (or , from the insert menu , Name-use in formula, i think this is the step in your version of office). At this moment, in the destination file A, on a cell you can type =F29FileB then press enter (use it in formulas, like =$C$2+F29FileB ). You will have the value from fila B in that cell, which will follow that cell, no matter how many rows you insert in source.