You would need to do one of the following.
A.) use one of the on-board zones (most receivers in this price range have a powered zone) for powering outdoor speakers. You would need to run speaker wire to the outside speakers, preferably conceiled and done to electrician's specifications. (a zone can be operated independantly from the home theater, meaning different source can be used, as well as output levels, typically they can be used simultaneously).
B.) use the zone preout to an external amplifier to do the same as I mentioned before, this relieves some stress from the receiver's amplifiers.
C.) get a wireless speaker system or wireless amplifer. I cannot recommend putting a wireless speaker system outdoors unless it is meant to be used outdoors. I cannot recommend placing a wireless amplifier outdoors either as they are not rated for outdoor use. You could place the amplifier as close to the outside as possible (away from temerature varients and moisture) and run speaker wire to the outdoor speakers, make sure the wire is rated for outdoor and in-wall use.
D.) this is perhaps the most expensive route is a network system (like Sonos) that would work on a similar principle to option C. but with the benefit of being part of your home network and it will have access to any and all music you want to put on either a media server or computer setup to run as a media server. Once such a system is established it is relatively easy to use, once again systems like these aren't intended for outdoor use (other than specific wires and speakers) so amplification needs to remain indoors. There are probably cost no object systems that can be placed outdoors, but I am not familiar with any.
