You can grant a user full control access to the user policy of a given web application using PowerShell with this script.  This script uses the following variables:

You can also grant different role types such as "FullRead", "DenyAll", or "DenyWrite" permissions as defined in SPPolicyRoleType.  For example use "FullRead" to grant you search crawl account readonly access.

$webApplicationUrl = "http://server" 
$account = "domain\user" 
$roleName = "FullControl" 
$webApplication = Get-SPWebApplication -Identity $webApplicationUrl 
$account = (New-SPClaimsPrincipal -identity $account -identitytype 1).ToEncodedString() 
$zonePolicies = $webApplication.ZonePolicies("Default"$policy = $zonePolicies.Add($account$account$role = $webApplication.PolicyRoles.GetSpecialRole($roleName$policy.PolicyRoleBindings.Add($role$webApplication.Update()