If your thought is to use both the interior (wired) speakers and some outside (wireless) speakers at the same time, I think you may end up being disappointed because there'll usually be a perceptible lag with the wireless speakers. If you're using only wireless it's fine since the lag is constant.
You could use an aux-to-bluetooth adapter coming out of your stereo... the built-in stereo won't send music OUT on bluetooth anyway, so you can link your phone into the stereo with the built-in bluetooth and the audio out to external bluetooth speakers through the adapter I mentioned.
There are readily-available wireless links like the Denon DN-202WT / DN-202WR but they're not particularly cheap, and you'd need power for it. I don't know offhand of readily-portable speakers using a non-BT wireless link, but there may well be some on the market. We just use a portable BT speaker outside driven from one of our phones, or link to the head unit in the Airstream when we're indoors or right by the trailer.
Zero Gravitas ó 2017 Flying Cloud 26U | Il Progetto ó 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. ó Sir Winston Churchill