Yes, only one dongle would be needed.
You then need a SS Hub and one-or-more SS controllers.
You could have someone flash the PIC for the dongle with pixelnet firmware for you, and also flash the latest versions of firmware for the SS Hub and SSC PIC(s) also. The PIC chips are fairly small, so if nobody local can do it for you ... you could mail the chips to a member willing to help and they would flash the chips and mail them back to you ... then you put the PIC into the stuff you built.
You set a jumper on the SS Hub which can enable 1 DMX512 universe [say channels 1-512 for DMX] on the SS HUB DMX OUT port, so thats like your DMX dongle now. Each SS Hub can emulate 1 DMX universe and the SS Hub jumper sets the starting pixelnet 512 channel range for that new DMX512 universe to use. They would each be DMX channels 1->512 and you would set the DMX stuff connected to them as needed. There are no DMX channels above 512, so each is a new DMX universe of 512 channels. So if you use one SS Hub, you can also have 1 DMX universe on the DMX OUT if you want and thats like a DMX dongle you didn't have to build.
You then would set it up as one giant 4096 channel pixelnet controller (in say LSP, running 4096 channels of pixelnet).
It just so happens that channels 1-512 are actually going to become DMX on SS Hub DMX out, and you would want to start your pure pixelnet channels at 513+ . Channels 1-512 would actually go to both DMX and pixelnet, but to keep it simple ... I would not double up unless you really know what your doing, so think of them as partitioned for simplicity.
Again, there are options galore ... but giving you the most basic way you could get it all working.
If xlights adds support for pixelnet, you could also just play LOR or VIXEN sequences and map out the channels to pixelnet accordingly or a mix of networks. Your hardware requirements would be the same.