Currently, there is no way to run WildStar via Steam while Steam is in Offline Mode.
If the Steam client is set to "Offline Mode" and you attempt run WildStar from Steam you will receive an error indicating "NC Platform Error (a02f00c8)"
To fix this, please do the following:
1) First, please close the WildStar Launcher
2) From the "Steam" menu in the Steam client, choose: "Go Online..." then click "Restart and Go Online"
3) Once Steam has restarted, start WildStar from within Steam normally