Have a look here for an assortment of bits and bobs....
http://www.talkphotography.co.uk/forums/showthread.php?t=244521
This -
http://i908.photobucket.com/albums/ac282/selectivefocus1/lbracket2.jpg - is one example of a brolly adapter. I have one that looks more like this....
http://cgi.ebay.co.uk/Flash-Shoe-Um...747?pt=LH_DefaultDomain_0&hash=item3a5c96439b
From the look of things I think the one Graham (FITP) sells is probably a better design because you should be able to position the flash head much closer to the centre of the brolly, and with better alignment towards the centre. The ball head fits to the top of the stand. There is a cold shoe to which you fit the flash directly, or the RF-602 receiver and then the flash, and a hole through which you can insert the stem of a brolly.
These adapters won't work with a softbox. The softbox will need to come with its own solution to supporting the flash, or you will need a different adapter onto which you can fit a more standard softbox. Here are examples I am considering....
Dedicated Speedlite softbox -
http://cgi.ebay.co.uk/60cm-strobist...graphy_StudioEquipment_RL&hash=item3cb102ae67 (with grid)
-
http://cgi.ebay.co.uk/24-60cm-Hot-S...graphy_StudioEquipment_RL&hash=item230a2c078d (without grid)
or....
Speedlite adapter to which you can fit a "standard" softbox -
http://cgi.ebay.co.uk/Bowens-S-Fit-...graphy_StudioEquipment_RL&hash=item3cb0e4523e
Bowens fit softbox -
http://cgi.ebay.co.uk/Bessel-60cm-x...graphy_StudioEquipment_RL&hash=item3cb1012323
The advantage with the Bowens speedring adapter is that you can fit all manner of modifiers - softboxes, octaboxes, snoots, beauty dishes, reflector and grid, barn doors etc.. The disadvantage is that such attachments are probably designed for (semi-)permanent use and not intended to be rapidly deployed and packed away. A dedicated Speedlite softbox might be lighter and hopefully speedier to set up and pack down.
I don't speak from a position of personal experience here, but from what I've seen I'm not sure that trying to make Speedlites work with gear designed for monolights is that the light pattern emerging from a Speedlite is quite different to that from a monolight, and I can see the prospect of major disappointment with a directed beam of light from a Speedlight, albeit quite widely spread, compared to the much broader radiance of light from a monolight. This beauty dish is an example that gives me cause for concern -
http://cgi.ebay.co.uk/46cm-Beauty-D...graphy_StudioEquipment_RL&hash=item3a5ee8b585. I'm pretty sure that you can see through the whole of the grid, but the illumination of the dish looks absolutely pants, and I think is caused due to the constrained path of light out of the Speedlite, which seems not match the lighting required to illuminate a beauty dish.
I'm considering buying some studio lighting, so for me I am umming and ahhing about whether to get a dedicated Speedlite softbox for convenience as a Strobist, or to go down the route of the Bowens fit adapter and a Bowens fit softbox. There's not a big difference in price but I'm still weighing the pros and cons.