You can use LDAP authentication with ProFTPd. Simply install proftpd-mod_ldap with

# pkg install proftpd-mod_ldap

afterwards edit your proftpd.conf file an enable ldap authen:

LoadModule mod_ldap.c
<IfModule mod_ldap.c>
    AuthOrder mod_ldap.c
    LDAPLog /var/log/proftpd/ldap.log
    LDAPAuthBinds on
    LDAPServer myhost
    LDAPUseTLS on
    LDAPBindDN "CN=mybinduser,DC=domain,DC=local" "mybindpw"
    LDAPUsers  "DC=domain,DC=local" "(&(sAMAccountName=%u)(objectclass=user))"
    LDAPAttr uid sAMAccountName
    LDAPAttr gidNumber primaryGroupID
    LDAPDefaultUID 65534
    LDAPDefaultGID 65534
    RequireValidShell off
    LDAPSearchScope subtree
    LDAPGenerateHomedir on
    LDAPGenerateHomedirPrefix /home
    CreateHome on
</IfModule>