Can you convert wired outdoor speakers to some kind of wireless relay to reciever?

while its overkill, check out the sonos system.
it will do exactly that and much more and sound good in the process (plus you can send the sonos music into your onkyo system and other rooms down the road)

Based on the OPs request this probably won't work. I'm assuming he has existing sources going into the Onkyo that he wants to output to Zone II as well as the main room where the Onkyo is. With Sonos he would need at least 2 players. A ZP80 connected to his Onkyo. This would connect to the Onkyo and take his existing source as an input. Then a ZP100 connected to his outdoor speakers.

The other issue is I'm pretty sure Onkyos can only output analog sources to Zone II. If for example his source is a CD player he need to connect it to the ZP80 and output the ZP80 via analog to the Onkyo. This brings in the subpar DAC in the ZP80. Perhaps for outdoor speakers the OP doesn't really care.

Of course if his "source" is files on a computer or NAS box a single ZP100 would work, but then his Onkyo doesn't fit into the equation at all.

Chicagotc- I think the sonos DAC's in this situation would work and sound just fine for an outdoor system.
I would guess he would probably need to buy the following:
Sonos bundle at $999
2 sonos bridges (1 only if he didn't want to get music from a computer source, 1 to place as an extender in the middle of the house if he has range issues)
That would be $1200 in parts but he might love the system and feel its worth it.

I agree, I have my ZP80 analog out to ZoneII on my Integra. It hasn't been warm enough for outdoor speakers here in Chicago yet,we had snow yesterday, but I'm sure it will perform fine for non-critical outdoor.

I'm a huge fan of Sonos and recommend it whenever possible, but it's still pricey.