Three ways to weaken spine on a carbon arrow -
1) Increase tip weight
2) Increase arrow length
3) Decrease nock weight
I don't know if Easton inserts are tapped all the way through like CX and GT inserts, but if they are they'll be standard 8/32".
If I remember correctly the Lightspeeds are .246 ID, so the GT adder weights would work - but be sure to check the specs on Easton's site. The weights just screw in behind the the broadhead with a long Allen wrench. They come in 5,10,20,50gr. On my target arrows I've added over 180gr behind the inserts, so they will work, and work well.
Depending on how far you need to go, though, new arrows or at least arrows that are longer from the start may be in order.