Unfortunately this is not currently possible, although this type of feature has been requested within GitHub issue #3854. If this matches your requirement, could you please give the initial description a +1 and add any context that is not already present as an issue comment.
Thanks @jrasell for pointing me to this “old” issue. I seem to have read that the management of secrets was going to be added to Nomad. I hope this functionality will be added at the same time.
Cyrille.