The assumption is that YKK #X zippers will mate with other YKK #X zippers of the same length and style. This only works for toothed separating zippers. You can not mate and separate coil zippers without doing a Frankenstien/Rube Goldberg arrangement. _If_ your current bag and the extender both have a toothed separating YKK #5 zipper of such and such a length in theory you will be fine. If not... with a toothed separating zipper of another brand all bets are off. It might work... it might not.
If you have a coil zipper on the current bag you can not use it at all.