Hi,
It's about to launch (i was told end of July):
https://jlcpcb.com/smt-assemblyFrom everything i've read here and playing around there are a couple of ways of using this:
1) Browse LCSC for stock, then search Altium GIT libraries for the same part
2) Using EasyEDA browse components using the "SMT" library option and drop them on a blank schematic. You can then export the schematics and PCB footprints.
Option 2) looks like the best way to go since you can check the stock levels and maybe the symbols/footprints/part numbers etc will be more compatible with their system.
I've created a template Altium project with JLCPCB rules, stack-up for 4-layer board and placed a bunch of common parts (0402, 0603, 0805 caps/res etc) on a blank schematic. Will share once i've had a chance to submit using their SMT line.