As of LCOS version 9.10 it is possible to use the Clustering Wizard in LANconfig to configure the WLCs all at once.
Note: With WLCs equipped with the "WLC High Availability Clustering XL option" you are able to select all of the listed WLCs and configure them all in one go using the WLC Clustering Wizard (see 1-Click WLC High Availability Clustering Wizard).