Can I use sprinkler cable for LED light strip extension?
The biggest barrier will be physical.  18 AWG solid is much too stiff to solder directly onto LED strips.  "The tail will wag the dog" as it were, and the strain placed by the wire on the strip will tend to tear the solder pads off the strip. 

I prefer to use #20 or even 22 stranded, however it's fine just to run that for a short 6-12" pigtail.  Then transition to the #18. 

Remember there is a lot more current on the common wire than the other 4.