As far as I am aware (and from past experience), this should be simply a matter of asking your designated Qliktech (or partner) account manager.
Enterprise licences can be clustered - but instead of several you have now, you will end up with one licence with a line entry in your LEF for 'number of cluster nodes = n'
Standard practice is to set up (add) and configure all of your cluster nodes from one of the servers (on which you deploy the QMS.
Note: Publisher licences can also be clustered in a similar manner.
It is a matter of contacting Qliktech and putting in place the needful processes to migrate all of your enterprise servers to the proposed clustered set-up - there has been no additional cost (at least for any of the migrations I have been involved in so far).
Hope this answers your question.