I'm fairly certain that the one spots aren't supposed to go over 8 connections because of a potential fire hazard if more that that are connected... Can't remember where I heard that though, it might be complete BS.
Actually, that ^^ is total BS.
So you should be fine with another extension...
Yeah, the OneSpot, while inexpensive, is far from cheap. It is much more than a typical wallwart. It is regulated and filtered with current limiting and short protection. You can hang as many pedals off the thing as you want and when it can't provide enough current anymore it protects itself by shutting down instead of burning up. It is, though, still a daisy chain which creates a ground loop and also can allow some really poorly behaved pedals to dump noise onto the power supply which might affect other pedals. Neither of these is a problem unless it is, of course, and if you're not hearing any unreasonable amount of noise from the chain, you're good.
The next step better is basically a parallel daisy chain with an individual cap across each "link". This can help keep some of that noise from a naughty pedal from effecting its neighbor, but doesn't help the ground loop.
Better still is actual transformer isolation of all of the outputs, so that they're not sharing ground through the power supply, and the loop is broken.
Better even is when each of the transformer isolated outlets is both regulated and filtered seperately. This pretty much ensures that one pedal can't fuck with another and should be the quietest possible solution. This is almost necessary when you're doing multiple different voltages and/or providing both AC and DC outputs.
For most things, a daisy chain works fine up until you run out of current. I don't even bother with regulated supplies for most things. Just one big filter on the DC input from a relatively hefty 9VDC supply and a smaller filter on each output has worked reasonably well for quite a while, except that this won't be protected from shorts or pulling too much current, and I have burned things. Be very careful of any "spare" connectors on your daisy chains! The outside of that barrel is the positive voltage, and if it touches the chassis of some pedal that's plugged into the same chain, it creates a short. The OneSpot will safely shut down, but other things might not be so well protected. At best, none of the pedals will light up (because they're getting like 0V supply), at worst the wallwart itself will light up...on fire.