I wasn't going to post a reply but you are being misadvised by others here.
If you are doing a KIT WINE then the "secondary fermentation" to which you refer is in fact just the continuing of the original fermentation after your have racked off the fermenting must from the initial deposits and fermentation is NOT YET Completed. This is normally done when the SG falls to about 1.010
It is important that you do no top up at this stage. The space above the fermenting must will still be filled with CO2 from the ferment and your fermentation lock will still be bubbling gently.
There is no problem with contamination by oxygen at this stage.
Now -- when fermentation is completed and NO gas is passing through the fermentation lock then you will need to rack off into a new carboy ( read your instructions carefully) and carry out certain procedures which depend on the type of wine you are making and indeed the type of kit
Above all read the instructions
You will have to go through a process of degassing, fining, and stabilisation. You should be topping up after degassing usually, since it is at that stage that your new wine loses its protective CO" blanket. I personally use a wine of my own making to top up to within about 3 cms of the bung. You can certainly use a commercially bought in wine to do the same. And I still use a fermentation lock to close the carboy.
It isn't difficult but......................
Read the instructions carefully
If it is at all a kit produced by a recognised maker then you will have all the necessary instructions to make all the right steps.
Above all don't panic. Despite what you will read in all sort of place the making of wine is not difficult and the wine itself is amazingly tolerant and robust during its early processes.
Good luck