If you are finding the alpaca slippery on its own, blending with any amount of the wool will help. Border Leicester's a medium wool so it will affect (effect? no English teacher in sight ...) the softnessof the alpaca. One of my "presents to myself" is sitting out in the garage -- a Cormo/Romeldale fleece in a soft brown, and an auburn alpaca fleece -- the present is to SHIP IT to a fiber mill and let them wash, blend, and card it. I'm dyeing to see how it turns out, but time is a precious commodity around here, so if I do it, it might be done by next Christmas -- the mill may have a 3 month backlog, but that's still before this summer!
I'd say how much wool to blend in depends on the crimp in the wool --you could get away with less wool in the blend with a high crimp wool (like my Cormo/Romeldale cross, I could go 10% wool, but in truth I love this wool too so mine will be probably 60/40) but with a lower crimps-per-inch you might want more. In almost any case, 50/50 would do it. If you can sample, you might try a few combinations from 10/90 to 50/50 and see which one you like. Or, if you want to preserve the hand of the alpaca, go for a finer wool like the Shetland.