Safest Way to Contain Spliced Lamp Cord
This family of lights uses all 120v wiring which means the full weight of NEC applies.  You cannot "freestyle" splices in the manner you are talking, you must use listed products, which those cheapie butt splices probably won't be.  They do exist, but O'Reilly Auto doesn't sell them and they don't use $5 crimp tools either.

So here's how you handle "the cord is too short".   Legally, splices can only be inside junction boxes.  The American gold standard way to splice inside a junction box is with "wire nuts" - for two 18ga. wires I would use the blue or gray ones.  Do not use stabs, they are not rated for 18ga.

If GE sells a listed method to attach their special connectors to your lamp cord, go for it.  Otherwise you can only use it between junction boxes, which means 2 junction boxes.  If you want to do it in one junction box, then buy 2 cords long enough to give you 8" slack inside the box, and splice there.

But I have to say: why on earth are you using this family of products? Don't get me wrong, GE's quality is first rate, but why wrestle with 120v products when you can get the same thing in 12v that's much easier to work with. 

Also, these prices are way beyond insane, at least 3-4 times what I've seen in quality 12v products.  IKEA has a broad selection of 12v products at sensible prices.  12v makes the products cheaper because they do not need expensive voltage converters in each lamp.    I buy cheap Chinese LED strips for $7 for a 16 foot roll, I mention that only to shake you out of a notion that $60/foot is a normal price for these things.