How can I charge secondary car battery?
Yes, you can use a diode between the car's electrical system and the secondary battery. In the worst case, this diode needs to handle the full output current of the alternator, so make it a beefy one.

Note that the charging voltage that the secondary battery gets will be slightly lower than optimum, but in practice, it doesn't make all that much difference.

However, if it worries you, and you have an alternator on which you have access to the actual windings, you can connect a second full-wave (6-diodes) rectifier directly to the windings, and then both the primary and secondary batteries will get the same charging voltage.