I like using fill flash for outdoor photos but I have a question about what it changes.
If I pop up my flash, and it is set to TTL, does it change my shutter speed automatically?
What is the proper way to set the flash to get fill flash and keep a higher shutter speed?
The camera, in P or A mode, will meter the daylight ambient, and will set the shutter speed for the ambient. The flash does not care about (is not affected by) shutter speed. However, the camera shutter will not sync flash faster than 1/200 second (D90). This is sort of a problem in bright sun.
So... the standard Sunny 16 rule says bright sun exposures will require near 1/200 second at f/16 at ISO 200 (hazy, cloudy, overcast, or shade will need more exposure). However the camera meters it, but that is about ambient. You cannot use 1/400 f/11 equivalent here because camera cannot sync flash faster than 1/200 second (maximum sync speed). The TTL flash has to work with that ambient setting, and it does work ... it simply sets its TTL flash power accordingly to be proper flash exposure. If it has enough power to pull it off.
The popup flash has very little power (guide number about 42), so its range as fill flash in bright sun is only a very few feet, more like 4 or 5 feet, nothing like ten feet. You will do much better using your Sunpak flash (decent power, it might make ten feet in this bright sun setting).
The popup flash will do TTL BL balanced flash, to be point and shoot flash in bright sun. I think the Sunpak will too, but I have no experience with it to be sure. In the unexpected case that it is not balanced flash (saying, if the subject is greatly overexposed in this situation), then use about -1.7 EV Flash Compensation on the camera to correct it. But I would not expect that will be needed, and expect balanced flash will pretty much take care of it. Practice a bit in the back yard before any big event where it needs to work right.
I have to mention that
Four Flash Photography Basics we must know - Flash pictures are Double Exposures is about this subject.
You mentioned "higher shutter speed". I think you mean this: Your D90 does have menu E5 to offer Auto FP, which will allow any faster shutter speed with certain flashes, those flashes which also support it. The camera popup does not. The Sunpak does not mention it, so I suspect it does not.
Nikon flashes like SB-600, SB-700, SB-800, and SB-900 do offer high speed sync FP flash (SB-400 does not). They will work with this D90 Auto FP option. Which such a compatible flash, then you could conceivably use f/2.8 and 1/4000 second with flash in bright sun, but at substantially lower power level (still, with those flashes, 10 feet should be possible).
See
Four Flash Photography Basics we must know - Auto FP and HSS about Auto FP flash options.
Again, the camera popup flash cannot do this option. My own notion is that f/16 1/200 second works OK for me. ISO 100 allows f/11 1/200 second.