How can I install soffit vents from the outside?
Are you planning on replacing the roof? If so, are you redecking the roof or just replacing shingles? If you're only replacing shingles then there will be little or no additional opportunity to install soffit vents. If you are redecking, make sure to use radiant barrier decking. It adds about $2 a sheet to cost and has a large impact on attic heat. No roofer will tear off and reinstall old decking.

Soffit vents vs gable vents vs whirlybirds vs vent fans will make little difference in attic temperature in Texas. I have had houses with all combinations, and no vents at all, and the effectiveness is as follows:


Soffit vents (almost no effect) 
Gable vents (little effect) 
Attic fan with gable vents (some effect, still easily 140+ in summer) 
Turbine vents with soffit and/or gable vents


I made the mistake of replacing turbines with attic fans on one house and the temp in the attic went up! 

You can definitely hire someone to install soffit vent channels, and they will come rake all of the insulation out of the way and install the channels, then move the insulation back. The impact of soffit vents will be minor though.

Honestly, if you are are not having moisture issues in your attic (i.e. you don't live in Houston) and you have a house built before 1970 in Texas with a $150 summer electric bill then you probably want to stop worrying about the attic and start looking at other systems for improvement. If you had no attic insulation then you probably have no wall insulation. Having insulation blown into the walls will have a noticeable effect. Have you replaced the windows with low-e vinyls, esp. on the south and west walls? That will also have an impact on cooling costs. After all of those, start looking at A/C upgrades, pressure testing (most utilities will do this for free or low cost). Soffit vent channels and retrofit radiant barrier will have negligible effect if you already have several ft of insulation in the attic.