Asset Management licenses per user

We have enterprise license keys for vendors such as Adobe and Microsoft.  These licenses are tied to specific Users.  How can I setup Asset mgmt to import and work with users?  It gives me compliance per machine and says I have say 100 licenses and 90 in use. 

However if a user leaves the company or is terminated how can I assign that license to another user? 

When the asset managers look at the list of imported license keys, how do they know which license key # to give to a tech to enter in the computer? 

Right now the import will only allow one version of the enterprise license key to be entered.  How can I assign this group/filter of users to that one key?