No. There is no wired output on the bar to connect the speakers.
I'm not Evensure the output for the SA5 would support those. However if you can connect the SA5 you would run into two issues.
Firstly it wouldnt give you rear sound for the soundbar. You would get surround from the bar and then stereo (front left and right) from the VIs.
Secondly if you have the 251s connected to the same SA5, they would be controlled as one one, so whenever you play through the VIs, it would play outside at the same volume
What I was saying is that it won't do what you wanted, even if you can plug them in.
The SA-5 is designed for music streaming to 1 zone. either with 1 pair of speakers at teh best quality or 2 pairs for better coverage - so if you have a large garden you can cover it with 2 sets of 251s. It will not control the pairs seperately.
As well as that, as I mentioned you won't get surround sound for the bar out of them anyway.
The only way to get surround speakers attached to the SoundTouch300 is using the WIreless modules that come with the Virtualy Invisible 300/700 or the Surround Speakers (essentially the VI300 anyway)